Requirement: Acquired some Luxury Cheese in Harva Town
After you got some Luxury Cheese from Harva Town you could technically challenge the Lemming Hero to a fight. If you win he will join your ranks and becomes a playable character. You can find the Lemming Hero on the little island to the east of the Cave of Chaos. You can also check #39 on the world map.
To lure him out eat/activate a luxury cheese and he will appear as the next random encounter.
If you just followed the walkthrough without much leveling in between then you are most likely too weak to win the fight. The Lemming Hero is at level 42 when you fight him and has some nasty one-hit kills and status attacks which keep you busy healing and resurrecting your characters almost each turn if you’re at a low level.
I beat him when I was at level 35 with my party. I guess you could beat him even earlier but I don’t see the need for that except if you are looking for a real challenge. So I suggest to challenge him when you are around the same level as him. Once you recruited him Hero is arguably one of the best damage dealer in the game. His ultimate skill does a fixed 999 damage!