According to its Bulbapedia page, it's very feisty and tedious.
Clefairy is often an unintelligent, lazy, loudmouthed, crass, vulgar, and generally rude Pokémon. He is typically selfish, often thinking about himself instead of others, and is also cowardly, preferring to win battles through the use of trickery, luck or other unconventional means. More often though he will try to flee and hide, sometimes even going as far as to disguise himself, to avoid battling others, especially against Pokémon who are by far larger and stronger than he. Clefairy would also rely on his signature item, the Poké Doll, for his ways of avoiding anything especially battles.
Clefairy does occasionally show some redeeming qualities, but he is usually a mischief maker most of the time. His manners and stupidity are often an annoyance to Red and the rest of his Pokémon team, and is also especially a major cause in fights between Pikachu and Tyrogue, who are both smarter, more well-behaved and otherwise more capable than Clefairy.
His mannerisms are also well known among Pokémon across entire regions, who typically refer to Clefairy as "that blockhead Clefairy". Rather than taking offense to this, Clefairy falsely believes it to be a compliment with the idea that he is "famous" among Pokémon.
To read more about him, click here.