Disgaea 3: Absence of Justice

You have to find them on certain weapons and subdue them. The max level you can have is a 100 Mediator. There are certain weapons they appear on check the board using the search function and all the answers because there are similar questions.

Actually what I've been doing is just leveling up important items. I started off with a Royal Orb that was legendary but a legendary Arcadia would be better if you have 1. Basically level up items (Legendaries become way more powerful of course but are much harder) and then reverse pirate them. Then dupe them ASAP (after attaching it to your best Emblem) till you get it to 100.

Originally after I leveled up the Royal Orb I did most of my duping on them until I had some good specialists. You know the professionals the staticians and all that maxed out and enough for each character... After that I went into a Makai Wars (best item I had at the time) and did the same there. My characters are much stronger now so I'm looking towards leveling up my Prinny Suit until I can get enough maps to get to the land of carnage.

It's best to work towards multiple goals at once. Which is why I am giving you so much information. The game takes enough time without doing things 1 step at a time. Organization and planning are key. Especially if you have class or a job with some down time that you can use to write out a game plan and do some math with.

No, they don't. Mediators show up only during Reverse Pirating

When you to Item World, and when you go to the Treasure room on floor 3 if you can get all the items in that room there is an chance to get an Mediator. Its the fastest an easy way to get them.

Mediator will sometimes show up when reverse pirating
if you kill them or have them on the boat when the last turn ends and have space in the item you will capture them
they will show up on the innocent list of a item and can be moved or combined