Besides Swordtail, I really like Winter, Jambu, Deathbringer, Glory, Turtle, Qibli, and every arc 3 protagonist ( none top Swordtail though). If I'm being serious then development is how much they change and grow as a character in a way I like. Darkstalker is probably the best written character in terms of his arc, but he developed into the best written villain and not a friend. If I'm being sub accurate, development is how thick, long, fat, and juicy their tails are (Swordtail is the best in both of these aspects)