Onyx has a foot long knife and Rhodolite has a staff. Their combined weapon is a spear.
Onyx was an assassin hired to kill an aristocrat called Rhodolite but couldn't and hid him away so they could be together. They were later caught out and Onyx quit being an assassin for him.
Onyx is alert and aggressive whilst Rhodolite is calm and passive. When they fuse they are more stable then when they're apart.
Onyx's gem is on his stomach and pulls out quickly and gracefully. Rhodo's is on his foot and he stamps the ground and it jumps out before he catches it.