Notes

Ranpo hates and doesn't believe adult except Fukuzawa (Bungou Stray Dogs novel vol. 3), so he never bother to reach the adulthood; and the main topic is here: how Ranpo get his partner?

This is purely a headcanon but based by canon informations.

Ranpo doesn't like to talk about heavy topic that doesn't have any relation with cases, mystery and puzzles. He just want to enjoy his life by doing any activity that he likes. Look at his acts and how he spends his days, nothing hard than playing and talking with other members of Armed Detective Agency. Is he even give a thought about romantic problem? My answer is he do, but never bring it out, sharing with other people. He just think, it's no need to rush about it. The time will come and he will accept it.

What is his type? How he notices her(I am using this pronoun because Ranpo is straight)? Let look into the canon informations. There's a Q&A by Mr. June Bride at a magazine; Ranpo, along with Atsushi, Dazai and Akutagawa had answered a few questions. I'll only write about Ranpo. There's a direct one, 'who is your ideal marriage partner?' Ranpo being Ranpo and answered: 'There's two kinds; first, a person who would /spoil/ me, and the other one is a person who spoil me /willingly/'. It's pretty clear, right? He accept a person whom love him and she will have a permit to pamper him, allowed to give him such an action that he doesn't allows everyone to. Had you ever seen Ranpo acted spoiled by a certain person? No. Not even Fukuzawa. What he allowed was a praise about his Super Deduction power, no more.

Ranpo also hopeless and he is romantic despite his behavior at the office. There's another questions at the same magazine; 'If your partner betrays you, what will you do?' His answers is simple: 'Cry'. What can I get from this little confession? He is very a touchy person. He said he didn't care with other's sympathetic story, but actually he do care about his own and I can imagine it how hard he tries to put those feeling aside when he out for mission and his partner is waiting for him at their house.