I think I already brought this up elsewhere, but I think it would be a nice advanced skill, maybe it has a high skill point cost so it isn't easy or cheap to get, or maybe it only becomes available after completing a special quest? And maybe successive ranks in that skill would reduce how much XP they leech from the player, or increases their XP bonus %? Would be nice to balance out the party and provide more support for the player's character, I think.