Silly Goat

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-Before we get to the defining early ritual texts, we must start with the brief mentions of Nezha/Nalakuvara in translated Buddhist scriptures.

-The earliest mention of Nalakuvara as the son of Pishamen, a.k.a. Vaisravana, appeared in the Northern-Southern dynasty translation of Buddhacharita (佛所行赞). Here, he is not refered to using transliterations that have "Na/Nezha" as its first two characters, but as Naluo Jiupo.

-In Bhaiṣajya-vastu, one section of Mūlasarvāstivāda Vinayapiṭaka (根本说一切有部毗奈耶药事), Nezha is brought up in a conversation between Sakyamuni and Ananda. Sakyamuni basically pointed at a mountain and said "100 years after my death, two brothers——Nezha and Pozha——will come along and build a temple here".

-Other scriptures described him as a Yaksha general (in 不空羂索神变真言经), a Devaraja (in 大佛顶陀罗尼), or a "ghost/demon king" (in 吽迦陀野仪轨).

I suppose you can find reasoning for all these readings in the Indo-Buddhist deity he's based on:

1)Nalakuvara the original is the son of Kubera. Vaisravana is either Kubera's epithet or his father.

2) In the Atharva Veda and Satapatha Brahmana, Kubera/Vaisravana rules over either evil spirits or Rakshas.

3) Yaksha/Raksha being among his subjects kinda carried over to the Mahabharata and Ramayana too, in which he's a wealth god who ruled over the city of Lanka, before his half-brother, Ravana, comes along and forced him to move out.

(Nalakuvara also had a wife in Ramayana. Ravana assaulted her, and he put a curse onto Ravana in response.)

4) Buddhism incorporated Kubera into their pantheon as a guardian deity under the name Vaisravana: here, he is the Devaraja of the North and lord of the Yakshas.

5) Because the Indian sources don't have a consensus on whether Nalakuvara's father is a Yaksha/Raksha or a god either, the translated scriptures also can't agree on the specific details of Nezha other than him being a guardian deity.

-It is in Tang Buddhist texts about Pishamen worship that Nezha's status as Pishamen's third son becomes more firmly established.

-However, there are actually 2 different variants of his relation to Pishamen: one said he was Pishamen's grandson, while the other said he was Pishamen's third son.

-The first variant is from Amoghavajra's translation of a ritual text, 北方毗沙门天王随军护法仪轨. In this text, he wields a halberd, "sees all four directions with his fierce eye" (尔时哪吒太子，手捧戟，以恶眼见四方), and will bonk monks and nuns over the head with his vajra club for harboring wicked and murderous thoughts.

-In this text, he is also described as a protector of kings and officials, a subduer of the wicked, and has a mantra one can recite.

-The second variant is found in two other Pishamen-related texts translated by Amoghavajra, 北方毗沙门天王随军护法真言 and 毗沙门仪轨.

-Apart from being Pishamen's third son, both texts also give him the role of pagoda-carrier. In the latter text, it is mentioned that the 21th day of every month is when he hands the pagoda back to his father.

-Another Tang source, 开天传信记, features a story where Nezha protects Xuan Lv, disciple of Śubhakarasiṃha, from a fall, introduces himself as Pishamen's son, and presents a Buddha tooth relic to the Buddhist master.

-He's described as a "youth" here, but honestly, 少年 could refer to anything between a young teen and a young adult. This story has an earlier predecessor in Ennin's travel diary, where he also mentioned Nezha giving a tooth relic to a monk.

-Finally for the Tang dynasty, there is a specific genre of Dunhuang paintings called "Heavenly King Pishamen arrives at Nezha's banquet". Usually, the painting is located on the west wall of the painting cave's antechamber.

[Mogao Cave 146, main chamber ceiling, northwestern corner. Nezha is likely the small figure on the left, clasping his palms together over his head.]

One Dunhuang text, the P.3564 Mogao Cave Merit Records from 933 CE, gives some context on the aforementioned genre: Pishamen goes to Nezha's banquet, but he either arrives late or not a lot of people show up, and Nezha gets so angry that he's ready to stab Pishamen with a sword, until Pishamen reveals that Amitabha is inside his palm and he cheers up again.

-Based on other inscriptions in Cave 72 where the "Nezha Banquet" paintings are titled as "Amitabha Enters the Pagoda and Arrives at Nezha's Banquet" and "Heavenly Lord Pishamen Invites Amitabha into the Pagoda for Nezha's Banquet", we can make some guesses about the above story.

-Perhaps Pishamen promises Nezha that guests/Amitabha will come to his banquet, no one shows up, thus Nezha gets murderously mad at him until Pishamen reveals Amitabha sitting inside his pagoda like a surprise present?

(Sidenote: later, Pishamen gets syncretized with the historical Tang general, Li Jing, and becomes a separate character known as "Devaraja Li". It wasn't a thing in the Tang-Song sources yet, though.)

-Nezha returning his flesh and bone to his parents is a thing that first shows up in Buddhist compendiums of the Song dynasty (景德传灯录，五灯会元) as a Chan riddle. Being a riddle, it just states that he returns his bones to his father, his flesh to his mother, then sits atop a lotus and preached the Buddhist laws to his parents.

-The "return of flesh & bone" may have been a derivative of Nezha's quote in the aforementioned Tooth Relic story, where he's like "I'll even give up my head and eyes, why won't I offer up [the relic]?"

-The way this riddle has been used, though, is very much centered around identity, seeing through the emptiness of the Five Skandas, and the delusion of the false self.

-There's also this Southern Song quote from 如净和尚语录 that described an opera performance scene where Nezha appeared on stage, likely performing the above narrative of "returning his flesh and bones" because he's described as taking off his clothes.

"…十二峰前上戏棚,那咤赤脱点天强。"

-Su Zhe, brother of the famous poet Su Shi, has an entire poem dedicated to Nezha. Here's a translation of the relevant lines, from Meir Shahar's Oedipal God:

The Northern Heavenly King had a son who was mad: He only venerated the Buddha—not his dad. The Buddha, knowing he was crazy and hard to be told, Ordered the father a magic stupa in his left hand to hold. Approaching the Buddha, the child bowed his head, Not unlike revering his dad.

-Aside from that, he's also known in Song sayings and poems for his wrathful demeanor——to the point of often having the adjective "Wrathful" attached to his name (忿怒哪吒).

-Fun fact, the discrepancy between Nezha having 6 arms or 8 arms appears in this period too, though 8 arms are the more commonly accepted version in textual sources.

-Lastly, a story from the Southern Song compendium Yijian Zhi mentions Daoist Master Cheng from the Mt. Mao tradition using Nezha's Fireball Spell to defeat a stone demon. This may be the first appearance of a Daoist-ized Nezha.

TL;DR: Nezha in the Tang-Song period was very much a Buddhist guardian deity who miiiight also be a Yaksha general, most well-known for his wrathful demeanor and multi-headed, multi-armed form.

The earliest mentions of the "Returning of flesh and bones" also don't have its later association with filial piety/father-son conflict, and is mostly a Chan Buddhist riddle.

Bibliography:

付方彦，《哪吒形象流变研究》

郭俊叶，《托塔天王与哪吒——兼谈敦煌毗沙门天王赴哪吒会图》

李小荣, 《那吒故事起源补考》

刘文刚，《哪吒神形象演化考论》

罗俊，《符号学视角下中国古代哪吒形象的演变》

任婧,《从无名氏到大英雄：论哪吒形象的演变与东传》

Shahar, Meir. Oedipal God: The Chinese Nezha and His Indian Origins. University of Hawai'i Press, 2015.

Nezha: 15 characters

-Li Gen the Yaksha (Qiankun Ring to the head)

-Ao Bing (pulled off steed with sash, Qiankun Ring to the head, tendons pulled out)

-Verdant Cloud Boy (archery accident)

-Yao Shaosi (speared)

-Colored Cloud Immortal (speared)

-Deng Zhong (knocked off steed with Qiankun Ring, then speared)

-Ji Li (speared)

-Li Qi (knocked down with ring, speared)

-Pong Hong (knocked down with ring, speared)

-Wang Bao (knocked off steed with ring, speared)

-Ma Zhong (incinerated by the Nine Fire Dragon Bell Cover)

-Long Anji (knocked down by a ring better than his, speared)

-Gao Lanying (didn't get her needles out in time, knocked down, speared)

-Ding Ce (killed by the Qiankun Ring)

-Lu Renjie (knocked off his steed by the Qiankun Ring in a fatal blow)

Note: I didn't count the Colored Cloud Boy, Lady Shiji's other disciple that gets bonked by the Qiankun Ring, because his death is never confirmed, even though he was already "struggling for his life on the ground" when Shiji comes out of the cave.

Yang Jian: 13 characters, 2 creatures

-Chen Jiugong (speared)

-Zhou Xin (Skyhowler'ed, then chopped in half)

-Bi Huan (Skyhowler'ed, then speared)

-Wen Liang (tried to escape after being bricked by Nezha, got hit in the shoulder by Yang Jian's slingshot, fell off his horse, and died)

-Yu Hua (Leizhen Zi knocked him off his steed, Yang Jian finished him off with his polearm)

-Zhang Kui's mom (indirectly, he tricked Zhang Kui into killing her via transformation arts)

-Chang Hao (Yang Jian transformed into a flying centipede, snipped its head off, reverted to his true form, chopped up the snake, then incinerated it with Five Thunder Arts)

-Wu Long (Yang Jian turned into a golden rooster and pecked it to pieces)

-Yang Xian (tried to flee after seeing Yang Jian turn into a tiger, chopped in half)

-Dai Li (Skyhowler'ed, then speared)

-Guo Chen (speared)

-Lei Kun (speared)

-The Nine-headed Pheasant Demoness (indirectly: Skyhowler'ed, then decapitated by Zhou soldiers on the execution ground under Yang Jian's watch)

Creatures: 

The Flower Fox Ferret, Mo Lishou's monstrous pet (Yang Jian burst out of its body after being eaten, tearing it into two halves in the process)

The One-horned Black Smoke Beast, Zhang Kui's magical steed (indirectly, same as Zhang Kui's mom)

Huang Tianhua: 7 characters

Chen Tong (Fire Dragon Dart sucked into Tianhua's flower basket, then decapitated by the beam of the Moye Sword)

The Four Mo Brothers (killed by the Heart-piercing Nail)

Yu Qin (killed by the Fire Dragon Dart)

Chen Geng (knocked off steed by the Fire Dragon Dart, then decapitated)

Yang Ren: 6 characters

Fang Yizhen (incinerated by the Five Fire Fan)

Li Ping (caught in the fan's AOE while trying to persuade Lv Yue)

Chen Geng (incinerated by the fan)

Lv Yue (incinerated by the fan)

Yu Xian & Yu Zhao (incinerated together)

Leizhen Zi: 5 characters

Xin Huan (Skyhowler'ed, then finished off by Leizhen Zi with a staff to the head)

Zhou Xin (Staff to the head)

Peng Zun (knocked off his steed via a staff to the shoulder, then decapitated)

Yu Guang (staff to the head)

Lei Peng (KO'ed with staff)

Huang Tianxiang: 4 characters

Feng Lin (speared)

Tao Rong (speared)

Gou Zhang (speared)

Gao Gui (speared)

Wei Hu: 4 characters

Yang Wenhui (Demon-subduing Club to the head)

Yu Da (clubbed)

Zhang Kui (clubbed)

Jade Pipa Demoness (indirectly: decapitated by soldiers under his supervision)

Jinzha: 3 characters

Wang Mo (immobilized by the Dragon Stake, then killed with a sword)

Yang Sen (immobilized by Dragon Stake, killed with sword)

Dou Rong (indirectly: Jinzha immobilized him with the Dragon Stake, but Jiang Wenhuan made the killing blow)

Muzha: 2 characters

Li Xingba (killed with one of Muzha's twin flying swords)

Lady Chedi (killed with twin flying sword)

...

Final Thoughts

-If you count the creatures, Nezha and Yang Jian's kill counts will be equal, though several of Yang Jian's kills are indirect.

-Jiang Ziya: tfw your logistic guy, responsible for overseeing the supply line, has a body count as high as your designated army vanguard.

-Yeah, Yang Jian's official job in the Zhou army is the Grain Transport Officer. I think it really fits his personality——the clever, ruthless trickster who's also a mature, polite disciple.

-Though Leizhen Zi's kill counts can't match Nezha and Yang Jian, there are multiple descriptions of him knocking down whole sections of the city wall with a single staff strike. Maybe he's the anti-fortification specialist of the team or something. /lh

-Huang Tianhua: the ranged attacker who keeps trying to be a melee DPS. /j

-For real though, he seems to rely a lot more on the Fire Dragon Darts from a slain foe, and when the Heart-piercing Nail is used after the Mo brothers' demise, it only ends up going straight through Xin Huan's wing and the Winged Immortal's right arm.

-Yang Ren always gives me the impression of a soft-spoken bureaucrat who gradually discovered his inner pyromaniac after receiving the Five Fire Fan.

-My list is only counting the named characters, but everyone on this list who's still alive during the Ten Thousand Immortal Formation Arc are described as unleashing their might and treasures during the battle.

-Considering the AOE effects of some of these treasures, they probably send a lot more filler NPCs into the Investiture. 

-BTW, there are 152 names on the Investiture who are labelled as having died in the Ten Thousand Immortal Formation. 124, if you leave out the 28 Lunar Mansions, who are killed by the 4 Immortal Swords seized from Patriarch Tongtian's previous formation.

-How many percent of those 124 kills each Chan Sect participant is responsible for is up to your imagination.

-Forget about the Nezha age discourse——Huang Tianxiang was probably about 10 when he made his first kill, since the text described his head as "still bald". If I remember correctly, in the Ming dynasty, before 10, most children had their heads shaved completely bald, or leaving only a few patches of hair tied up in a braid/bun.

-He's also 14 when he made his third kill, and 17 when he was executed by Qiu Yin. Four chapters prior, Huang Tianhua died to Gao Jineng.

-Jinzha and Muzha: tfw your total kill counts combined is only a third of your little brother's.

-The one detail I've observed is that Jinzha ends up rescuing/attempting to rescue Nezha, Huang Tianhua, Jiang Ziya, and Tianhua again (together with Muzha) after they get knocked down.