If the Imp dies, another player can summon a new one before the first player's Summoning Sickness expires.
The Imp will level up a few kills after the character does. The Imp can sometimes level up before the character does (that is, be a higher level than the character), but that's very infrequent and the mechanics are unknown.
The Imp increases in size as it increases in level.
This item is not usable in Solo Quests, including Dwarven Boxing and Bonus Mission Pack missions.