Malamar is the evolved form of Inkay, and the Pokémon makes its debut in Pokémon Go's Psychic Spectacular event. In it, Inkay will rarely appear in the wild, but will likely appear in raids and field research activities. For players lucky enough to catch enough Inkay to evolve into Malamar, this Pokémon is an interesting choice for the Grand and Ultra League. This guide explains in detail how Malamar is good and how to best use it in Pokémon Go.
Malamar is a Dark and Psychic-type Pokémon. It will be weak against Bug-type and Fairy-type moves, but is resistant to Psychic-type attacks. You have to be very careful when using it as it is only resistant to one type of attack, especially with how weak it is to Bug-type moves. There aren't many Fairy types you need to worry about in Pokémon Go, but there are a lot more Bug-type Pokémon and moves floating around in the game. If Malamar is cornered by a Pokémon that has access to this attack, you are in trouble.
When using Malamar in PvP, he has a maximum CP of 2.359, an attack of 151, a defense of 142, and a stamina of 162. He can do well in both the Grand and Ultra League. He will use the same move set where he uses the fast moving psycho cut, then super power and foul play loaded moves. He is a solid striker with the highest defenses but has a good amount of stamina to stay alive decently enough in a fight. With the correct combination, you can expect to see him frequently in future PvP matches.
It will likely play the role of Pokémon Lead, Switch, or Closer. It can be in essentially any position on your Pokémon roster, depending on which team you use. Of the three choices, it's a stronger Lead and Closer Pokémon due to its decent bulk, especially if you want to use it in the Ultra League, and it has XL candy.
Overall, Malamar is a highly recommended choice that won't break any records or change Pokémon Go's PvP meta. But it'll be a reliable choice if you ever use it. However, you have to be careful of any Bug-type Pokémon.