PLATONIC:

"Re:Viled—Starting Life With The Cursed Saint From Zero" by xtenn8: Reinhard is the widely hated Cursed Saint, quiet and resigned to his fate. Heinkel is a controlled mastermind, and Subaru, Rom, and Felt get trapped in his web. Tagged as slowburn friendship and found family.

"When Your Friends Finally Get Tired Of Your Family Drama" by Only_Slightly_Obsessed (A_Stressed_Cupcake): Knight squad calls out Wilhelm's behavior toward Reinhard.

"Decompression" by Only_Slightly_Obsessed (A_Stressed_Cupcake): Second part of a 2-part collection. Read "Peer Pressure" first to get the context. The second part just has more Reinhard. Several people get tortured! And the aftermath of that.

"On the Cusp of Tomorrow" by Avaricious_Trash: Reinhard notes Subaru's suspicious behavior and ends up learning about RBD.

"Breezy" by Turcoverdin: Reinhard dies for a moment and ends up in a strange place with...Subaru?

"Fate's Promise" by petalil: Platonic soulmate AU where Rein, Ferris, Subaru, and Julius are all soulmates and have the power to appear as time-traveling ghosts up to a certain point.

"(k)night(s) out" by NightsMistress: Subaru nearly dies but then gets to hang out with friends yay!

"Heavy Endings Burn Bright As Flames" by PricklyLamb: Subaru has some complicated feelings about Reinhard being able to die

ROMANTIC:

"Fate, Worse Than Death" by Turacoverdin: A popular fic for a reason! And has a great grasp of Rein and Subaru's particular habits and idosyncrasies. A chance DP acquisition spirals into Reinhard learning more about his friend than he ever would've imagined. Suffering abound and Subaru deals with the Mortifying Ordeal of Being Known.

"The Demon Who Wasn't" by Snor2: Reinhard is the least demonic demon out there, and these two idiots have no clue what they're doing (You can skip chapter 2 if those tags are not your cup of tea).

"32" by Honcamaron: A fantastic introspective fic of Subaru wondering about if he wants to take his and Reinhard's ambiguous relationship to the next level and if Reinhard even wants him (He does, you idiot).

"Covert" by Harperd: Moments between Rein and Subaru after they confess to each other at the start of Arc 5 (They write a TON of these two. Check out their other fics)

"my sin, my pride" by an anonymous author: This nameless author has a TON of romantic stories of sizable length for these two, and this one in particular deals with Subaru being isekaied as the Archbishop of Pride...and immediately giving up any evil by getting into a marriage deal with Reinhard. Please check out their collection this is under called "grocery list." Also trends more towards writing SubaRein than ReinSuba.

"Prideful Cinderella" by Pinkshuchan: Pride brings back Natsumi Schwartz to infiltrate a ball and meet Emilia. He instead ends up spending an unexpected amount of time with the Sword Saint (Hasn't been continued for a while, but I very much still enjoyed what was there!)

"Meant to be a Graveyard" by Zeivira: FemBaru is taken by Regulus instead of Emilia and Reinhard intervenes, catching a glimpse of some concerning behvior.

"What If the Stars Could Dance? Would You Dance With Me Instead?" by RavenCentauri: Reinhard has a stressful conversation and gets some well-deserved affection from his mutual crush.