In the Gokudou-san series:

V.1 - Gokudou-san wa Papa de Aisaika
Sachi is the physician of Amamiya clinic. One day, suddenly his yakuza childhood friend Kengo told him to raise Mushi, whose birth was kept secret. Thereupon, for Mushi’s sake, he pretends to be Kengo’s lover and lives with him?! He hates Kengo, who’s a yakuza, but… As they taking bath and going to amusement park together like a real family, the bond gets strengthened…

A love story between the yakuza, egoistical childhood friend and the beautiful, tsundere physician who’s doting on the child.

V.2 - Gokudou-san wa Kyou mo Papa de Aisaika
Now that his love for his childhood yakuza friend Kengo is requited/mutual, Sachi started a new life, in which he, Kengo and Fumi, the kid with special circumstances, live together.
The bond between Kengo and Sachi deepened as they became lovers, but Fumi got into a quarrel at his nursery school over Fumi…
Furthermore, when a beautiful woman named Misuzu, who helps the group and gets along well with Kengo, showed up, Sachi began to question what his existence means to Kengo and Fumi, and noticed his insecurity…

V.3 Gokudou-san wa Aisareru Papa de Aisaika
Yakuza Kengo and his childhood friend, doctor Sachi, are lovers. Due to some circumstance, Kengo was raising Fumi as his child, and the three of them have gotten used to living together. Every day went by peacefully, but suddenly Gino, a Sicilian mafia man who claimed to be Fumi’s uncle, showed up! Gino wanted to take Fumi away with him, but Sachi resolutely opposed that. On the other hand, he was worried about the fact that Kengo wasn’t Fumi’s real father… What should they do so that Kengo, Sachi, and Fumi to be able to live together as a family forever…?

V.4

Yakuza Kengo and doctor Sachi, his childhood friend, are lovers. They live together with Kengo’s “son” Fumi. One day, a beautiful person in Japanese clothes named Tsukasa of Sanohara-gumi brought a baby to their house to seek refuge. Inukai, the lawyer obsessed with Tsukasa also appeared, and the baby was caught up in the commotion…?! Meanwhile, Tsukasa said that Kengo is cold-hearted, but Sachi didn’t deny it…