There are a total of 3 Jester fights in the game. The first is mandatory and is inside the portal in the same room where you fight Gigapede in one of the earlier missions.
If you're as stupid as me though (and also haven't played the game before) then you can miss the other 2 fights. The next portal is literally right before you fight Geryon (the horse) and is next to a divinity statue before the stairs you go down. This is mission 12 I think. The last one is a couple of missions later (the mission after you fight Lady) and is in the room with three trials after you've gone up the God Cube chamber. These portals are really easy to see, but I stupidly ran past them oblivious to what they were and thought nothing of them. Silly me eh?
The fights themselves aren't really all that hard. You can mostly slap your combos on him, but the thing you need to dodge is when he summons the bouncy ball things that fly everywhere. Just keep moving and avoid them until they go away. The missable fights are the same but towards the end Jester will go on a giant ball that shoots things at you occasionally. Just keep going with the shooting/sword hitting and you'll get there.