The supporting character's harem is very normal - Chapter 211 Do you like me?

A skilled thief gets a system in an Alternate Wizarding World. The system seems useless in the beginning since he hasn’t unlocked any features. But…

[The very first feature of the System has been unlocked.]

[Congratulations. You’ve unlocked the class feature. Due to your existing skills and experience, you have gained the Special Class "Arcane Thief" as your starting class. There’s nothing an Arcane Thief can’t steal. The more you steal, the more your Class will progress.]

[The fields of expertise include:

— Stealth Magic
— Wards
— Traps
— Enchantment
— Pickpocketing
— Knife/Short-Sword Wielding]

Wait…this is just the first feature?

The vast Kunwu Mountains, one hundred and eight thousand. Spiritual ethereal sword rising, sweeping the summits of the nine provinces.

A modern day otaku is reborn in a wuxia world, becoming the little shishu of a little sect. Unexpectedly discovering he has the martial arts trade system of a garbage web game. Through it he can actually exchange for all of the martial arts in Mr. Jin Yong’s novels.

And so, Nine Yang Manual, Nine Yin Manual, Nine Swords of Dugu, Eighteen Dragon Subduing Palms… All kinds of amazing martial arts started to dominate the jianghu of another world…

- Description from Novelupdates

Fate had never been kind to Ji Ning. Wracked by illnesses and infirm his entire life on Earth, Ji Ning knew early on that he would die as a teenager. What he didn’t know was that there really was such a thing as life after death and that the multiverse was a far larger place than he thought.