Gugu/Relationships

Fushi
Gugu initially doesn't see the immortal as anything more than an annoying kid in the shape of a human, and is jealous of Fushi when Rean shows an attraction to Fushi over him. But after some time, when Fushi proves that he does care for Gugu by saving him from being kidnapped and even overcoming his fear of forests for his sake, he starts seeing him as his "monster brother". He even fights alongside Fushi against the knockers with the two making a wonderful team. When he dies protecting Rean, Fushi is clearly heartbroken at the loss of his adoptive brother and even goes as far as to use Gugu's form to prevent Rean from knowing about his death.

Rean Cropp
When Gugu first met Rean, it was by returning her lost dog which she repaid him by giving him her a very valuable ring, which could have saved him from his life in poverty. Later, he saves her from a log rolling down a hill that ends up crushing his head, and giving Rean a small scar on her arm. After being patched up by Beer Gramps, he still retains his crush on Rean and when she comes to the shop he gets extremely bitter and jealous because she shows more romantic attraction towards Fushi than him. He and Rean tend to fight a lot, but the two do start to grow a budding romance, with Rean not knowing that it was Gugu who saved her from those falling logs.

After the two become teens, Rean invites him and Fushi to her birthday party, where her parents are trying to arrange a marriage for her. Gugu tries to put his feelings aside, knowing that Rean has to marry nobility since she was a noble and just wishes for her to be happy. But when her house begins to fall apart after a knocker attack, Gugu uses his body as a shield to protect her from the rubble, and Rean opens his mask and kisses him before his death.

Booze Man
Booze Man is the one who found Gugu, saved him, and gave him his mask as well as put an extra organ in his belly that stores highly concentrated alcohol. He sees Gugu as his own grandson.

Shin
Shin and Gugu used to be very close as brothers, but one day, Shin betrayed Gugu by taking all their savings and leaving Gugu to fend for himself. The two meet again in an alleyway when Shin is thrown out of a door and lays on the ground, seemingly unconscious. Gugu gives him Rean's ring to help him live a better life, but he did not forgive him (Shin asks if he's Gugu, to which he responds, "I don't have a brother").

Four years later, Shin comes to Booze Man's shop to meet Gugu again. Shin clearly has trouble keeping the conversation going while Gugu keeps silent. Finally, he cries out, "What the hell are you here for?! Did you come here just for me to tell you how worthless you are?!". Shin tries to apologize, but Gugu keeps going, saying that the Shin who betrayed him was "the real you". Shin tries to show that he has changed by showing Gugu Rean's ring, but Gugu takes the ring and throws it into the grass to which Shin jumps forward to catch it. Despite Gugu's obvious disgust towards his brother, Shin insists that he will always be there and invites him to live life as a family again. Gugu, on the other hand, doesn't want his brother to take away any more of his "precious treasures", the people around him.

In Gugu's vision of paradise, Shin carries him on his back. This proves that, despite his anger towards his brother, Gugu wanted to make peace with him.