The Greedy Golett is mostly a joke character that was introduced during the Unova arc.
There's not much to say about. He keeps searching for money, and always talks in a creepy monotone. He spends most of his time stalking people and asking them to give him money.
Abilities in battle
Hasn't been shown battling much, so his abilities in battle are unclear.
He first appears while the group is exploring Dragonspiral Tower, following them and asking them to give him money. He spends some time after that stalking the trainers, until Tagg catches him. He is later shown in the same jail cell as Ghetsis, asking him for money. The Greedy Golett later returns in the Hoenn arc, where he participates in the Pokeringer with a Hydreigon named Sunshine Smiles. It is currently unknown when, where and how the Greedy Golett and Sunshine Smiles met.
He later reappeared in the Unova 2 arc, where he offered to help Stalker!Frillish follow Kohai in exchange for money. He later protected Stalker!Frillish from a Dream World Pokémon, doing so because Stalker!Frillish still owed him money. Stalker!Frillish then developed a crush on him, and "broke up" with Kohai. The Greedy Golett and Stalker!Frillish were last seen flying above Unova on Sunshine Smiles.
In the AU
The Greedy Golett is known to have evolved in the AU and appears to have amassed a lot of power.