Below are my main hororen headcanons. I'll add to this as I think of them.

horohoro

MBTI: ESFP
Top Love Languages: physical touch, acts of service
Sexuality: (disaster) bisexual, male leaning
Other Details: ADHD, PTSD

ren

MBTI: INTJ
Top Love Languages: quality time, physical touch
Sexuality: gay
Other Details: suicidal ideations, PTSD

during the shaman fight

• Most of my headcanons for this time are on my timeline.
• During the downtime between arriving at Patch Village and leaving for Tokyo Island, Horo and Ren grow very close and begin to develop feelings for each other.

post-canon

• Ren desperately wants to stay away from the Tao estate and insists of revamping the China Wok restaurant in order to stay in Japan longer. His father allows it, but only for a limited time.
• Horo visits Ren as often as he can, be that in Tokyo or in China, even making up bullshit reasons to visit. It's obvious why he visits, but Ren pretends he doesn't know and just complains about having to see Horo so often.

sexuality & bedroom dynamics

• Horo is a service top and Ren is a power bottom, but on the dom/sub scale, they're both switches.
• Horo is desperately horny for Ren but too proud and stubborn to take action, so he spends a lot of time - and I mean A LOT - jerking off while thinking about Ren. This definitely starts during the Shaman Fight, and Ren pretends he doesn't hear it. (He definitely does hear it.)
• Ren is incredibly controlling and likes to boss Horo around and treat him like a dog. Horo insists he hates it, but it actually turns him on (and he hates that it turns him on).
• Ren is super sensitive, especially his nipples and his scar, and if Ren is turned on enough, Horo can make him cum just from touching and licking his scar.
• Horo has no self-restraint and struggles with reining himself in. He is both a desperate puppy, eager to pleasure and obey Ren, and a greedy, wild wolf, too horny and desperate to fuck Ren to stop himself from losing control.
• Ren insists he doesn't like it when Horo loses control because it means Horo constantly disobeys his commands, but he's actually insanely turned on and can cum untouched from Horo fucking him so roughly.
• Horo is incredibly jealous and possessive, especially of how many people look at Ren and want him, and he must constantly stake his claim on what belongs to him. He leaves a bunch of bite marks and hickeys all over Ren's body and Ren fucking loves it.
• Ren loves to go down on Horo and watch him fall apart when he cums. It makes him feel powerful.

funkuro high school au

• Ren's father wants him to participate in a club (like chess or something like that), but Ren doesn't want to do anything his father wants.
• Horo is in the gardening club and takes it very seriously.