Support for Dantés?

Waver-Helena-Scathach/Atalanta would work.

Can never go wrong with Hans, he works with everything. Alexander is also pretty good, being the only silver servant with a quick buff.

Beyond that, typical supports like Waver, Helena, and Caster Gil still provide a lot of benefits for him. Even Merlin provides good utility for Dantes. Right now though, Scathach, Atalante, and Alexander are the only servants with quick card buffs.

As stated, anyone who can give him a boost to quick cards like Helena or Scathach or NP works well. Scathach has the same deck as him so quick chains and a targetable quick buff. Her np also stuns which can help stall alongside Dantes’ np drain. If you know how she works to generate np or stars (using her buster cards) she’s a good offensive support. And with all the stars that can be generated with this team she shouldn’t have an issue critting which vastly improves her performance. Jack also for obvious reasons. Jeanne Santa Lily can also be a decent offensive support. She can improve his star gen, heal him and should be NP5. So a NP chain with her and Dantes should deal good damage, especially since her NP also increases attack for that turn. Carmilla can help with quick chains but is more of a independent unit. However her utility skills have synergy with Dantes’ (np drain, defense down & star gen per turn). I wouldn’t recommend 3 stars like Ushi, Medusa or Alexander bc they’ll take all the stars. Ceasar could work tho. Then there’s the generic Hans, Mash and Waver who work with anyone.

Helena, Hans, or even other normal option as for supportive Casters to support him as main DPS

Dantes is also not bad as semi-dps to generate star for your main DPS. You can have team like Ataltane/Shishou/RIder Kintoki+Dantes+supportive caster. I think.

Waver, Merlin, Hans, Mashu, Helena, Caesar for general teams.
Helena, Atalanta, Scat Lancer, Alexander, Caesar for more specific Quick teams.
Crit star generator : Fuuma, Jack, Cursed Arm, Jing Ke.

For example, Quick servant like Kintoki will appreciate Hans Crit Star gen and Crit buff along other amazing buffs Hans can offer and Helena is the main Quick Support on the party.
Hans give party Crit Star drop buff, Atk Up, Crit Up and then Kintoki Rider buff himself with his own skill, Helena give Kintoki another Quick buff and other cards buff as well.
At least for up to 3-5 turns, everyones on your party has high chance to crit.

It's pretty poweful for agressive gameplay, as you can see there's no hard defense such as evade or Invincibility in these kinds of party. Kill as fast as you can because that's something that Quick can do.

Waveclearing and farming Quick team : Scat ass or Quick AOE DPS + Helena + Waver, Atalanta/another AOE DPS or Alexander in the backline. Use Caesar if you're clearing a node with large HP pool enemy waves with more than 100 K HP, he can give you damn strong party steroid.
If you've NP gain CEs, NP battery CEs like Halloween Devil or any similar CEs, you can use Quick back to back NPs like everyones saw on YouTube.

Right now the only Quick card supports are Scat Lancer, Atalante and 3* Alexander. In the future, before Skadi, we get Assassin Wu Zetian to the list.

Aside from that, Hans and Waver work for everyone. Their massive boosts are universal.

Caesar, a Quick Saber with NP dmg up, attack up, and crit dmg+stargen up is also good offensive support.

Mash solves his low survivability with targettable invuln+def up with a dmg cut included (post-camelot), and a taunt.

If you want to use him as main dmg dealer, critical star gathering CEs+avoiding riders is recomended. And a critical star generator unit (Dantes alone contributes, but is not enough for critfests).

Alternatively, Halloween Princess/ Holy Night Supper/Knight Marines/Heaven's Feel CEs (Sakura one, freebie, not the 5* Justiceae version...), if you want early AOE NP, and more powerful.

Azure Magical Girl/Little Halloween Devil if you want to do the NP many more times in a fight.