tsuzuki's parents are unknown. there's no indication of who is mother might be, but there is speculation that perhaps hakushaku is his father, based on hakushaku's hair and some of his commentary regarding tsuzuki. we know for certain tsuzuki had an elder sister, named ruka, but there's barely any information matsushita has given us on her, either; there's only been implication. a common theory is that ruka died of an illness, such as tuberculosis, due to tsuzuki mentioning he watched a loved one waste away while he was helpless to stop it.
“how did he turn out so much like me?”
a young woman involved in a case tsuzuki and hisoka were working on. she was very sick, but it was her wish to be able to win a balldancing competetition before her death. according to hisoka, tsuzuki fell in love with her after being her dance partner. for her sake, he made a deal with hakushaku to extend her life to the end of the competition. something to note regarding hisae is her design is similar to storybook ruka's.
a young shinigami and tsuzuki's current partner. this relationship could be construed as platonic and familial, but... the love between these two is a major appeal. hisoka is willing to cease existing if it means he'd be without tsuzuki. hisoka has stated he views tsuzuki to be his entire family, and tsuzuki reciprocates these feelings, even going so far as to tell sohryu he cares more about hisoka's wellfare than the people of gensoukai should war break out. tsuzuki has even, in his own way, stated that hisoka's presence at his side has given him joy.
“you have to be happy first before you can share your happiness with other people, i think?”
- tsuzuki, offering hisoka half of a mooncake2
no, i'm not joking. this is an actual implied thing. going back to hisae for context, there's a point where tsuzuki tells hisae she must "dance like you want/love1
me." when asked who taught him how to dance like that, he says it was his sister, and hisoka even questions what kind of family they were. also going back to hisae, as stated in her segment, her design and storybook ruka's are very similar. speaking of the ruka design, in the storybook she's meant to be a genderbend of tsuzuki - who casually mentions he would get in a relationship with himself. when she sees tsuzuki the first time, hisae mistakes him for her brother - just like he could have found her having a resmeblance to ruka. there's also a segment in the catholic school arc where tsuzuki says he's had a "forbidden love," which could certainly apply to incest. albeit, there could be another explanation for that line...
tatsumi: the secretary of the office, and a former field partner of tsuzuki's... but implied to be more. while he can be harsh with tsuzuki, it's also apparent he has a deep affection for him, and will not hesitate to fight for him. there's another parallel in the storybook arc, where tatsumi is stated to be in love with ruka - who, mind, is also essentially tsuzuki. tsuzuki, for his parts, goes to effort to ensure his book counterpart and the book version of tatsumi are together. it's implied tatsumi couldn't handle tsuzuki's mental health issues, and therefore backed out of their partnership... but that doesn't mean there's not affection between them. tatsumi, for his part, has given hisoka full blessing, if not outright instruction, to take care of tsuzuki in his stead. it's possible the forbidden love tsuzuki was speaking about in the catholic school arc was referring to a relationship with tatsumi, another man.
1 this is a specific word in japanese that can be both want and/or love. either way, it indicates deep intimacy, like that of lovers
2 gensoukai, where this exchange is taking place, is based off of ancient china. in chinese culture, mooncakes represent togetherness, and completeness by being with loved ones; this happens shortly after tsuzuki and hisoka are reunited. hisoka, for his part, shares the mooncake with tsuzuki :)