Finally, in the home stretch! Last set of proper gym leaders to date, until Scarlet and Violet drop!
Milo's name in Japanese is Yarrow, which is still visibly the basis of his Galar written name on his card and his uniform. And Yarrow is a common herb with a long history of medicinal uses. The Common Yarrow's scientific name is Achillea millefolium, where they might have gotten "Milo" from for his English name.
Nessa's name is Rurina, nickname: "Ruri," from ruri-nigana[瑠璃苦菜], meaning "Lapis Lazuli(colored) Ixeridium dentatum" it doesn't appear to have any common names in English. Not sure where they got "Nessa" for the English.
Kabu's name is kabu. Kabu[蕪] is "Turnip." It refers to the same Brassica rapa as Candice's(Suzuna) name, although hers uses an older outdated kanji. Also has a homonym thing with Kabuto[兜] the traditional samurai helmet which is also incorporated into the fire(火)type Iogo on his jersey
Bea's name is Saito referring to saito[菜豆]: "Kidney bean"/"Haricot." Her English name is also Bea(n).
Allister's name is just Onion, which I find really funny. Possibly alluding to him being a crybaby.
Opal's name is Poplar, like the tree.
Gordie's name goes back to using Japanese; Makuwa from makuwa-uri[真桑瓜] the "Oriental Melon." It ties into...
...Melony's Japanese name just being Melon(Meron[メロン]). I just kind of assume they had the Winter Melon in mind specifically, from her hair color and the ice type.
Piers' name, Nezu is from nezu[杜松]: "temple juniper." Like Clair, I assume this is meant to reference both the spiky leaves/needles as well the dark colored and sometimes toxic berries. Also homonym with nezu[鼠] meaning "rat" but can also refer to a certain dark gray or black color(I assume the color of rat fur) which sort of plays into the dark type too.
Raihan's name is Kibana from kibana-fuji[黄花藤] the Laburnum anagyroides, "Golden Chain/Golden Rain." Same family of plants that Drake's name is based on. Although Drake's association with it seemed a little weaker, Raihan has some unique added context; his league sponsor is MC Bank, which I assume is, along with the Golden Rain/Chain, a play on a Dragon's hoard of gold.
Bede's Japanese name was just Beet(biito[ビート]) as in the vegetable.
Marnie's name in Japanese was just Mary[マリィ], presumably from Rosemary, as it doesn't seem to relate to any plants on its own otherwise. As I recall, back before the game's release had been cause for a lot of speculation about her and Chairman Rose.
Figure I'll throw the expansion (ex)gym leaders to round things out, although half of them aren't that interesting or complicated.
Klara's name is just romanized from Kurara[クララ] and kurara[苦参] is the name of the Shrubby Sophora. Various species of the plant have uses in traditional Chinese medicine, although appropriately for Klara's type specialization, the plant's roots are poisonous.
Avory's just cropped from Savory(Seiborii[セイボリー]) the herb savory, of family Satureja.
Mustard is Mustard(Masutaado[マスタード]) sounds like Master(d) and obviously the mustard plant. (Oh and his wife's name is Mitsuba[ミツバ] from mitsuba[三つ葉] the japanese wild parsley, aka honeywart.)
Peony was also still just Peony(Pionii[ピオニー]) as a kind of joke on being the brother of Rose. His daughter Peonia's name was Shakuya[シャクヤ] from shakuyaku[芍薬], the chinese peony(Paeonia lactiflora).
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