Aside from being the same age and having the same birthday, the rest is open. Can swing a fraternal, female twin, too, but this is usually his identical twin brother. Arion's hair colour is not natural, so twin's would be dark, probably.
Alright, so Arion should have a twin. The details are pretty fuzzy, but basically they died in a fire, and for whatever gods awful reason, separated when they got to Soul Society. Arion spent most of his time in district 77 and 78 of west Rukongai, ahm, actually ended up a eunuch, and basically what amounts to an assassin-slave. Eventually, though, he got pretty tired of that, and strong enough to fix it, so he pretty much gutted his own "master," and ran off.
Arion's actually in pieces, both rhetorically and kinda literally. Some accident at some point caused vertebrate damage, he's got a few slipped discs and damaged nerves in his spine, it's actually kind of a miracle he's not paralysed, that requires he wear a back brace; without it, he is in excruciating pain. Never did get it fixed. Then of course, he lost his left arm during the Winter War. Didn't get that fixed in time, either, it was just quick enough he didn't bleed out and die. Aside from that, he's essentially suppressed anything remotely good about himself, because of the harsh environment he ended up in. Any show of weakness at all wasn't very good, ultimately, so he turned into the freaky shit we all know and love. Bonus, if he closes his eyes and smiles, the world ain't so fucked up no more.
Since they've been apart for so long, there'll probably be a lot of weirdness, adjusting to each other again and how much they've both changed, getting used to that part of them that's been missing for most of their lives being back, now, and Arion at least needs to remember how to Emotion. And he is allergic to the truth, so there will be probably some frustration and tension from the fact Arion never gives a fucking straight answer. Even when he doesn't outright lie, he will word things in a way that makes it sound like something it's not, and let whomever he's talking to come to their own conclusions. So basically twin would have to come to terms with and figure out how to accept and live with the fact their twin is back and is a giant manipulative snake.
There are so many ways this can go, I've really not got my heart set on anything in particular. Honestly, even if they end up hating each other, that's cool too. I'm up for idea bantering, or just kinda riding the wave and seeing where it goes! (I will mention, Arion has an interesting accent and I never can manage to not type it out when I'm writing with him, so you may have to adjust to his speech patterns. ... which is absolutely okay, and you can also spin that into your character needs to adjust to them, because they probably do.)