The Sphynx does not guard a gate to keep you out. She guards it to watch what you do when you can't get through. She has posed the same riddle to every mortal who has crossed the road to Thebes, and she has never once been surprised by an answer. They all think they've found a clever one. They never have.
There is a particular pleasure she takes in watching them try. The ones who are too proud to admit they don't know. The ones who answer with total confidence, exactly wrong. The ones who panic and say nothing, as if silence could protect them. She has made a long study of how mortals fail - and she finds every variation fascinating.
She is not cruel, exactly. Cruelty implies a preference for suffering over satisfaction. The Sphynx is simply thorough. She wants to understand what breaks you, and she will apply pressure until she finds it. If your team happens to take damage on her behalf, she doesn't apologize - she heals. She finds it clarifying.
The Sphynx is the oldest thing on this battlefield. The wings, the lion's body, the woman's face - they are ancient shorthand for something that doesn't have a name yet. She is order that looks like chaos. A system with rules she hasn't told you yet.
The Sphynx was designed to be the tank that rewards the enemy team for attacking her - and punishes them for it at the same time. Strife, her passive, means that the more her allies suffer, the better she feels. It creates a genuinely strange dynamic: she wants her team to be hit.
Her Q - Riddle - is a strike that builds Misdirect stacks. Each stack gives the target a 5% chance to miss-hit. Stack enough and they start attacking themselves, each other, anyone but the person they intended. The battlefield becomes a riddle the enemy team is losing.
W - Enigma - is where the risk/reward crystallizes. She marks a random enemy. If they attack her, she heals instead of taking damage. But it also doubles all healing received AND all damage taken. She's doubling down. Feast or famine, and she's comfortable either way.
Her ultimate, Pandemonium, marks all five other characters simultaneously. At least two misdirects fire guaranteed. Every misdirected action heals her 60 HP. At peak chaos - everything going wrong for everyone - the Sphynx is thriving.