Renji is a lieutenant because Byakuya chose him to be his lieutenant, just like all other lieutenants gained their positions. He deserves his rank. He has regularly been fighting opponents that are stronger than him, which naturally makes him seem weaker than he is, but he is strong nevertheless. He has a Bankai. He was beaten by Byakuya (a captain), Szayel (an Espada with hard to counter powers, who stopped him from using his Bankai), Yammy (the highest ranked Espada), Aizen (no need to say anything there). He defeated Yylfordt, Sado & Uryū.