Kurokami (Black God): Excel

Excel, (real name: Maria) was originally in a contract with Steiner until his death.  Afterwords she makes a new contract with Mikami to get revenge on Reishin.  Her contract with Steiner had an unusual side effect of slowing down her aging and she may be hundreds of years old.  Because, of this she is a very skilled contractor and comes off as unusually mature.