Quetzalcoatl still hv her role with my existing lineup?

TL;DR: My (shitty) evaluation is Jack of All Trades, But Still Pretty Good. Merlin maximizes her effectiveness.

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Quetz plays the hybrid-role of a Critical Buster Servant with supportive abilities in her targettable Buster buff (doubling as a targetable Guts) and a higher Charisma rating than Ozy. Her one-turn-lasting 3rd Skill, which is jam packed with her Critical Damage and Absorb skills (and Riders already have the highest Star Absorption rate across all available Servants on NA), allows for some nasty damage, on top of also being a 20% battery that scales to 30%

Rider Kintoki may compete for her position as a boss killer, since Kintoki also features a battery but charges to 50%. Considering you got all copies during Rashomon, his NP Damage likely outscales Quetz's Level 1 NP. Both of their Card Boosters feature the same minimum and maximum Damage Boosts.

On the other hand, you may prefer Ozy for a supporter due to his team-wide battery and being the only Servant to boost Skill Buff Chance Up (plus his Charisma), while also being a good damage dealer due to Imp-Privilege.

It kind of boils down to team composition. Is there a boss with a huge HP pool, and you can field really nice Star-gen? Are you gonna roll for Merlin and run double Garden + Hero Creation (even just one is good enough)? If you like to see huge yellow numbers, Quetz is a really good, if not the best, choice.

You're good and Ozy NP 2 won't dissapoint you.
I think 3 ST riders, especially Kintoki and Ozy is more than enough to do the job.
You don't have any hole in the rider department and i would say adding her to your Chaldea is for collection or waifu.

If you really like Quetzal so much then you can roll for her, though it's up to you.

She still has place due to her 3-rd skill is quite good to catch a opportunity for a amazing critical NP-x-B and destroy many enemies. Off course that skill is somehow 1 turn lasting but 6 turn CD but still great. That also make more flexible opition for you to choose a critical team (like pair with Raikou)

Generally Ozy are more fitting for a team, but Quetz is more like a answer to give a final shot to your enemy. Iskandar, Medb, Mordred are good but they need special team composition. Kintoki is really great but Quick-oriented self-sufficiency which is somehow not compatible with other riders (either you will use him heavily, or not use him rather than choose Ozy+Quetz)

Anyway, if you consider you have Ozy then wont roll for Quetz, that is also reasonable.