Klee

Klee is a high damage dealing, and heavy hitting pyro elemental character. She uses bombs as her primary attack that deals Pyro damage so you can consistently put fire on your enemies, making it very easy to chain it with other elements. Her elemental burst deals high damage that automatically attacks nearby enemy so you can still do your other attacks while it's active. Her utility also extends outside of combat since she also comes with a useful unique passive that displays locations of nearby resources (unique to Monstadt) on the mini-map. Very useful during exploration.

Lost Prayer to the Sacred Winds is the best weapon that you use for her since it increases crit rate as the secondary stat. Solar Pearl is a good second option that you can obtain outside of gacha, otherwise you can craft mappa mare. For the artifact sets, go with 4 piece Crimson Witch of Flames for more Pyro damage. Pyro damage % bonus in cup with crit rate, or crit damage, for the crown.

If you only want to build her as secondary damage dealer, or a support, you can use 4 piece Noblesse Oblige artifact set, or 4 piece instructor. 4 piece Wanderer's Troupe, energy recharge set (2 piece exile + 2 piece scholar), or atk increasing sets in general are also not bad options. Use Prototype Malice for the weapon for some extra heals.

You'll want to bring her with another Pyro character for the elemental resonance, increasing the team's ATK by 25%. Xiangling or Bennett can be good options since they can increase ATK, just through different ways. Then you can either add some supports like Venti for utility from his elemental burst, or Beidou for elemental reaction from her burst. For the last spot, Jean is a solid option for an offensive support that can be a healer as well, since her elemental skill can collect the scattered mines and launch them.

"Normal Attack
Throws things that go boom when they hit things! Perform up to 3 explosive attacks, dealing AoE Pyro DMG.

Charged Attack
Consumes a certain amount of Stamina and deals Pyro DMG to enemies after a short casting time.

Plunging Attack
Gathering the power of Pyro, Klee plunges towards the ground from mid-air, damaging all enemies in her path. Deals AoE Pyro DMG upon impact with the ground."

"Jumpy Dumpty is tons of boom-bang-fun!
When thrown, Jumpy Dumpty bounces thrice, igniting and dealing AoE Pyro DMG with every bounce.

On the third bounce, the bomb splits into many mines.
The mines will explode upon contact with enemies, or after a short period of time, dealing AoE Pyro DMG.

Starts with 2 charges."

Klee's Blazing Delight! For the duration of this ability, continuously summons Sparks 'n' Splash to attack nearby enemies, dealing AoE Pyro DMG.