Sezaki Azumatsu

Sezaki is from another hidden village, from a noble shinobi family whose kekkei genkai is very rare. He was born a girl so his family kept him confined to the estate with the intention of marrying him off as soon as he got old enough. Sezaki decided he did not want to spend the rest of his life stuck in a house popping out babies so he ran away. While on the run he disguised himself as a man and was like wait this actually feels great. He's a mix of self-taught and taught from shinobi he met in his travels. Eventually he found Konoha and was allowed to join, and then joined ANBU. He wears a deer mask and goes by 'stag'.
His kekkei genkai works by him 'infecting' a target with his blood, just a little splash on the skin works, but the more the less chakra it takes. He can do healing with it: stop blood loss, knit cuts back together, repair internal damage without surgery, etc. Or he can do massive internal damage, pull all your blood out through your pores, cut off circulation to particular areas, etc.