A Certain Magical Index/Characters
For a visual aid for character relations, please check this map, and for characters' power levels in relation to one another see this one. Naturally, beware of spoilers, especially if you aren't up-to-date with the light novels.
For consistency's sake, please use Western naming order convention (first name then surname).
WARNING: SPOILERS AHEAD. UNLESS YOU ARE UP-TO-DATE ON THE LIGHT NOVELS, PLEASE TREAD CAREFULLY.