Quirk: Fat Mates- Upon one time, unknowing activation, the Quirk fattens the 'Soul Mates' of the user, the weight slowly increasing at set rate per individual, regardless of diet, and cannot be lost. Fattened Mates, in addition to increased fat levels, have increased strength, durability, speed, and their Quirk might be enhanced depending on the Quirk. User knows who a Fat Mate is when they see a Mate have a pink aura.

Weakness: EVERYONE, excluding the user, have no idea that the Quirk exists, making the world thinks its normal, along with making the user essentially Quirkless afterwards.

(Nobody bats an eye at the women gaining onscene amounts of weight for no reason, and continue on as normal.)

When Izuku was diagnosed with a Quirk, he was confused for over 10 years when he couldn't figure out what it was. Come the day of Mt Lady's debut, he figured part of it out right there, considering Mt Lady was very, very fat regardless of her size.

(And she gave him a long kiss/make out session before sticking him DEEP in her giant, massaging cleavage to drop him off at school.)

10 months after training for, and recieving One for All, Izuku figures out with 100% certainty what his Quirk is after Ochako Uraraka, a friend he hasn't seen in over 10 years catches and kisses him even deeper than Mt Lady, while appearing MUCH fatter than Mt Lady.

His Quirk made him a harem of fat girls and women, all of them weighing a ridiculous amount. Making the best of his situation, and deciding he deserves something nice for once, Izuku embraces his fattened harem with open arms, getting into various hijinks due to their increasing size.

Harem Includes

Ochako

Momo

Mt Lady

Mina

Kyoka

But Not Limited To

Midnight

Himiko

Melissa

Mei

Mirko