This is your annual Tsumitsu propaganda! Let's see what they've got going on.
First of all, they're very tactile. They've hugged each other more than once, and Tsukasa literally collected Mitsuba's body, lol, holding him in his arms like a princess, patting him on the head, petting him, hugging him, and snuggling. Tsukasa even makes a little protective motion with his hand.
Tactile on every level, Tsukasa literally touched Mitsuba's soul, repeatedly touching it, scraping his bloody fingers across the mirror that held the unborn Mitsuba. Sweet!
Tsukasa is proud of his creation, calls it beautiful, and presents it as the pinnacle.
Secondly, Tsukasa sheltered Mitsuba in his boundary when he couldn't be in his own. Tsukasa placed Mitsuba's mirror right in the middle; they were literally roommates. Tsukasa also dresses Mitsuba (the red scarf of fate!), feeds him, and cares for him when he's sick.
Tsukasa also always asks what Mitsuba wants and gives it to him - strength, life, friends, he gave him a whole new world in the literal sense.
By the way, have you considered the possibility that Tsukasa might agree that Mitsuba "can become a real human" because Tsukasa himself once longed to return to humanity and live with them, already half-supernatural, and he knows those feelings? He supports his protégé's similar aspirations. I also don't think Tsukasa recreated Mitsuba purely as a joke, but that's a long story. You can read about it in my analysis! If you're interested.
They have intimate sparkles between them
Thirdly, Tsumitsu spends a lot of time together, enjoying it! They play like normal schoolchildren, which is very important for bonding.
Tsukasa brings Mitsuba to his friends without leaving him alone, which is also important for the beginning of friendship.
Tsukasa says he likes Mitsuba, quite frankly, using "suki doumo," which can be found in the original or in anime dubs that don't deviate from the original. "Suki doumo," of course, simply refers to the level of regular "suki," which is used both romantically and in a completely platonic, familial, or friendly way. Tsukasa says he likes him "very much." I'm not saying this is said in a romantic context, but from friends to lovers is the most reliable relationship development, and Tsumitsu have it.
Funny detail: Mitsuba recognizes Hanako's name from Nene, he already heard it from Tsukasa, Tsukasa yappied him a lot!
The icing on the cake is the photograph of the red house. Mitsuba has a direct connection to Tsukasa, through time and space; he was destined to die and meet him again from the very moment he connected with the red house. Of course, I have more lore explanations for why this could be, but even with the plot hole, the fact remains. Something even on the level of soulmates, on the level of Nene's time travels to little Amane.
And lastly: despite all this, they are still very fucked up! Isn't it wonderful? Lol 🌈
