If a creature that attacked once attacks again in a second combat, it gets an additional +2/+0, for a total of +3/+0
P/T Set Number 12 Language Portugus Legal Formats Commander, Legacy, Modern, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Pauper, Pioneer, Standard, Timeless Rulings (6/18/2021) If a noncreature spell was already cast by an opponent the turn Esper Sentinel enters the battlefield, that opponent already cast their first noncreature spell this turn, and Esper Sentinel's ability won't trigger for that opponent that turn
So if we leave the realm of five colors, who do we have
Being an un-counterable 5/5 flier is a decent start, though the mana cost is tricky