Pyllis

Easy to Build

It's hard to have easier gear requirements than Pyllis. Build her with Defense. Then build her with more Defense. Have you built Defense on her yet? Good, then it's time to build her with some more Defense.

Teamwide Protection

Every action Pyllis takes (and several she doesn't) work to keep her allies alive. Her S1 Provokes, forcing enemies to attack her and preventing them from using their strongest attacks. Her S3 grants all of her allies a barrier, protecting them from damage. And every four times Pyllis takes damage, she gives her team a Defense buff, further mitigating any damage they might end up taking.

Dark Elemental

Being a Dark-elemental tank is a very good thing (why do you think Fallen Cecilia is so good?) Being Dark-elemental means that no Fire, Ice, or Earth hero ever has elemental advantage on Pyllis, and it also means that her Provokes never suffer from elemental disadvantage, even against Light-elemental heroes.

Good Damage Ratios 

She's never going to be a primary damage dealer, but Pyllis' damage can be surprisingly solid. Her damage scales off Defense (did you build Defense on her yet?) and the cooldown on her S3 is a mere 3 turns. Being Dark elemental, she never suffers elemental disadvantage against targets, and she also does bonus damage to Light opponents, further increasing her potency.

Low Investment

As a 3★ hero, Pyllis uses Stigma instead of Molagora to enhance her skills. This makes it easier for her to reach her maximum potential than it is for 4★ or 5★ heroes, who are heavily gated by the rarity of Molagoras.

Vulnerable to Defense Penetration

Since Pyllis scales so heavily with Defense, she typically builds more Def% than she does HP%. This can be a problem when she encounters bosses or enemy heroes who can ignore Defense with their attacks, such as Ludwig or Watcher Schuri.

Terribly Slow

Making up for her top-tier HP and Defense, Pyllis has absolutely abysmal Attack and Speed. The low Attack isn't a problem, but the low Speed definitely is. If Pyllis' enemies are getting two turns in for every one of hers, it's very difficult for her to keep up with their damage output, even with her heavily defensive kit.

Outshone By Fancier Tools

Pyllis' kit is effective, but it is also bare-bones. She can't make her allies Invincible, or revive them from the dead, or Provoke all her enemies at once, or negate enemy attacks with Skill Nullifier, or one-shot an enemy once she gets low on Health. The problem is, this is the kind of thing other heroes can do. Pyllis' kit is always helpful, but it doesn't have the breadth of power and versatility that some other heroes have, which means she can't always handle the crazy things Epic Seven can throw at you.