Monica can be found in the Castle of Loanne. She won't join you if your main character is Mikhail. If you are Julian, the only way to get Monica is to not go through with the Shipwreck Scenario. If Monica is in your party, don't attempt to get Mikhail otherwise he'll force her to leave.
Monica is meant to be a main character. She's very customizable and will become good at anything she trains in. You can adjust her stats to make her good at whatever you want be it magic, swords, or anything else. As she is, though, she won't excel in either.