Backtagging: always & forever
Threadjacking: when appropriate (ask if unsure)
Fourthwalling: why not? just check first!
Anything else?: ♥
Kissing/hugging/etc: platonic hugging, yes
Fighting: he loves fighting, especially for fun
Injuries: ask first
Mindreading/memory wiping/etc.: ask first
Anything else?: bear in mind his age, but otherwise, if you're unsure of anything, inquire within!
❇ Notes on appearance, for those encountering him: Sephiroth is very pale, but not bizarrely so. More unusually, especially for someone of his age, his hair is silver. Perhaps his oddest feature are his eyes, which are bright green with a faint glow and have pupils like a cat's.
❇ For those with special sensitivity/perception: Individuals who can sense such things might be able to sense the entity Jenova, who is present, in a sense, within Sephiroth, though dormant. His genetic structure is stabilized, so there is no present danger of Jenova taking control or expanding her influence, but she is there.
❇ Warning: As Sephiroth only exists as he is via an abusive act and has been raised as an experiment, playing the character may touch on some issues relating to abuse and mistreatment of children. I try to be sensitive, but if anything ever makes anyone uncomfortable, just let me know! I'd much rather be informed than unwittingly upset someone.
❇ Information: For more about Sephiroth at this less common canon point, you can read an application here.