Leo

Fast

Leo needs to take the first turn in a PvP match in order to disable the enemy team effectively, and thankfully, he's naturally quite quick, which makes this easier to accomplish.

AOE Stun (and Speed Break)

Need we say more? The ability to completely disable the entire enemy team for a turn is a game-changer. That Leo's S3 not only causes AOE Stun but also Speed Break doubles the blow -- not only does the enemy lose a turn to Stun, but it'll take longer than usual before it's their turn again. Heroes that use Spd to calculate damage, such as Vildred or Cidd, even suffer a DPS loss in the bargain.

High Debuff Chances

After Skill Enhancements and without any Effectiveness whatsoever, Leo has:

• 35% chance to Stun on his S1
• 100% chance to Defense Break on his S2
• 75% chance to Stun or Speed Break on his S3

Those are very high chances to land debuffs, especially debuffs as devastating as Stun and Defense Break.

Easy to Gear

Leo is stunningly easy to gear for PvP (pun not intended). He only cares about two stats (Spd and Effectiveness) and his best Artifact is the humble 3★ Cursed Compass. On top of that, Leo doesn't even need any Molagora to get his job done. Skill Enhancement ups the effect chance on his S1 and S2, but his S3 only increases from Awakening him and that's the skill that actually matters. Get him some good boots and the Compass, Awaken him, and he's ready to go.

Needs Strong Allies

Leo has roughly the same offensive and defensive capabilities as a package of tissue paper. If he is outsped by the enemy team, it is very unlikely that he will survive long enough to use his debuffs, and if he somehow happens to be the last hero standing on your team, he has no damage potential with which to eke out the win.

Pure Stunner

The only reason to bring Leo to a fight is for his AOE Stun. Not only does this mean that he is hard-countered by common PvP tactics such as Immunity sets, heroes with AOE Immunity skills, or bad RNG, but it also means that he is basically useless in PvE, where bosses are immune to his Stunning antics. (He can also cause Defense Break, but so can other, more PvE-appropriate Earth-element heroes such as Helga, Falconer Kluri, Bellona, and Iseria.)

Elemental Disadvantage

Being Earth-elemental means that Leo struggles to disable Fire-elemental heroes such as Lilias, Aramintha, Kayron, or Tamarinne. Because Leo's worth lies entirely in his debuffs, this can spell trouble when, say, Aramintha dodges his AOE Stun and responds immediately with one of her own.