Solo Leveling System with Harem Upgrade (SPOILER ALERT !!!)
Possible Harem / Difficulty
Tsunade Senju / Med
Mei Terumi / Med
Ino Yamanaka / Low
Hinata Hyuuga / Low - High
Hanabi Hyuuga / Med
Anko Mitarashi / Low
Kurenai Yuhi / Med
Kushina Uzumaki / Med
Mikoto Uchiha / Med
Samui (Do not know the last name) / High
Kaguya Otsutsuki / Extreme
Fuu / Med (Comment by DemonKingVan)
Sakura Haruno / Med (Comment by DemonKingVan)
If you want somebody else say in the comments, I'll try to see if it's viable. I must say that the Kaguya was a comment that suggested and well ... It's a challenge so I'll try.