Dahlia

In short: Dahlia is the game's first Hydro shielder. He protects the team and speeds up the active character's attacks, but deals almost no damage. His shield and ATK SPD buff scale with Max HP, and his Burst is the core of his kit, so his build is HP and Energy Recharge.

How he plays: his Burst, Radiant Psalter, deals AoE Hydro damage, grants the team Favonian Favor and creates a shield based on Dahlia's Max HP that absorbs Hydro damage with 250% effectiveness. While Favonian Favor is active, a broken shield can be restored for 1 Benison stack. Benison builds when the active character lands 4 Normal Attacks, and his passive grants 2 stacks at once for Freeze — up to 4 per Favor. His Skill is mostly there for energy particles.

ATK SPD: while Favonian Favor is active, the active character attacks 0.5% faster for every 1000 of Dahlia's Max HP — up to +20% at 40k HP. So he wants partners who attack a lot with Normal Attacks.

Stats: HP 40k+ (the ATK SPD buff cap), Energy Recharge 200%+. Crit Rate only with the Favonius Sword.

Partners: he fits Freeze best, with Skirk or Wriothesley: Freeze builds Benison quickly, while Escoffier and Furina cover damage and healing. He also works in Normal Attack teams with Yoimiya, Wanderer or Ayato.

Constellations: C2 is the best stopping point: a restored shield is 25% stronger. C1 restores energy per Benison, C4 extends Favonian Favor by 3 seconds. C6 adds +10% ATK SPD and revives a fallen character once every 15 minutes.

Weaknesses: beyond the shield and ATK SPD he offers little, he doesn't apply Hydro off-field, and outside Freeze and Normal Attack teams he falls flat.