As a Gastly, he was shown to be quite gullible and would fall for all the Haunter's pranks. He was also scared of ghost-type (despite being one) and with the aid of Clarissa evolved into Haunter.
Gengar is not usually serious. Compared to its closest friend, the responsible Alakazam, it comes across as immature and mischievous. He does like being around Alakazam.
Despite his immature side, Gengar does know when to be serious and is very close to Clarissa.
Trivia[]
Gengar is the only Ghost or Poison-Type owned by Clarissa
He is the first Pokémon Clarissa has ever traded away