Laharl is the main character of Disgaea: Hour of Darkness.
Even after Laharl is crowned the new Overlord half way through the game, he is still referred to as "Prince" by Etna and all those who still remain in the Overlord's Castle.
In Disgaea 2: Cursed Memories, Laharl appears briefly to try to force Etna back to the castle. He even tries to steal the main character spotlight by fighting Adell and his cohorts. He is playable after completing the optional missions "I want to fight an Overlord!" and "Summon Laharl!" He joins the party after he fails to convince Hanako to become his chef.
In Disgaea 3: Absence of Justice, Laharl finds Mao and assumes that he is the legendary Overlord. He also blames Mao for stealing his height and screen time in the game. Mao also thinks that Laharl is the legendary Overlord and they fight to see who will claim the title. Laharl, Etna, and Flonne join the party to help Mao battle against Baal.
He also makes appearances in Phantom Brave and Makai Kingdom as a hidden boss. In Phantom Brave he is trapped in a bottlemail and constantly tries to make Ash and Marona his vassals. In Makai Kingdom he is brought to Overlord Zetta after he defeats Flonne and Etna. He then tries to show who is the superior Overlord.
Disgaea Dimension 2
In Disgaea D2, a sequel to the original Disagea, there's a subplot in which Laharl and Etna are cursed, with Laharl becoming a female version of himself and Etna becoming a "2P color palette" version of herself.