The problem lies with your bf who isn't doing his job as a parent. It's his role to teach her how to dress appropriately (can you imagine the teasing/bullying she probably has to deal with at school?), proper hygiene and self care, and encourage her to at least try new foods. What's he waiting for? Kids don't raise themselves ...

I had to laugh at the Harry Potter robe. My guess is that if you went to the school, you'd find other children with daily outfits too. Maybe not Harry Potter, but sport teams, other characters, or certain outfits. I had one who would only wear a certain type of shirt, and the other would only wear shirts of certain sports teams for awhile. The Harry Potter thing will pass.

For mine, if they wanted to only wear the same outfits, I insisted they be clean, and taught them how to use the washer and dryer. If they didn't have enough for a load, they had to add other family laundry. Win for me as most of it ended up being done by them.

The hygiene thing lays squarely on her bio parent. He needs to parent.

I do not think you should have to spend time with your SD if you do not want to. That said, a frank conversation with your BF about hygiene (especially after puberty) and the impacts it can (and will) have socially. Part of our job as partents is to help our children become socially acceptable so they can find their own friend group, jobs, family etc. Ask him if he would hang out with her if he was 10 and not her dad. How would other 10 year olds respond? Wearing the Harry Potter robe sounds like a comfort peice of clothing - like self-soothing. Might be worth suggesting counseling, maybe even family counseling.

Accepting it...might take disengaging from the situation. 

My SDs didn't dress weird but the were (are) extremely picky about clothes and food. After several years of helping with clothing I stopped because the parents didn't care. They could be wearing clothes a size too small and with shoes falling apart and only I would do something to correct it. Note also the SDs never asked either, they weren't taught how to take care and speak of their own basic needs.

I helped a lot with food too, they had not been introduced to food at the right times, or at all. They ate all white food with only few exceptions. At 7 & 9. I changed that successfully with YSD with the 'how to introduce food to toddlers ' process. It failed with OSD because I think she has Selective Eating Disorder. Anyway I didn't disengage completely until YSD announced she was a vegetarian for no reason, and she actually is a vegan but doesn't admit it. So DH has to cook and do the shopping when she's here. She's 17. I explained to him why very calmly and he agreed.

Hygiene..,the battles! Awful. They had never learned to use a washcloth or wash their own hair (they just didn't at DHs and BM didn't make them either or did it for them). I taught them to wash, brush their teeth, do their laundry. YSD loved this. She still doesn't shower enough or brought her teeth enough but she keeps neat and doesn't reek. She's smelled once or twice after coming from BMs but I kept the car window down and told DH. We had some other incidents during COVID which you can find on my blog. OSD rebelled against any hygiene requirements. DH let her so I stopped trying. I disengaged. I would not sit next to her or take her anywhere in public if she was dirty or smelled. I would not have her in my car. It was a hard line. DH only started trying with her when HE had to smell her or her breath.

Basically you won't fix it. You can only ignore or disengage. Figure out how you can remove yourself from it. Read the disengagement forum.

I still don't accept it, it's still aggravating with only YSD17 around and having no say over it, but I've come to terms with disengaging.

What is white food? Like only things that are white in color? Does this include cauliflower or literally just bread and pasta? 

How do you accept it?  Well... I guess the real question is morally how could you look the other way and date someone you know is neglectful of their child? I  mean.. why would you want to.. seems very unattractive to me.

I'm not saying kids will not be weird..or imperfect.. but your BF refuses to address real concerns.. that are going to make his child's life very hard.. why? lazy? stupid.. neither of those are great characteristics in a mate.

Does she get bullied, and have you asked him about that aspect?

When my daughter tries to evade brushing her teeth I say fine, up to you but don't come crying to me when kids at school tell you you smell.

My GD 'marched to a different drummer".  Odd clothes, non-conformist type. Bath avoidance.  Food fetishes.  This is the one who had speech apraxia.   Some people are just different.  She's 31 now, lesbian, drives a truck, seems happy.  She's a very sweet, empathetic person so if she's happy, I'm happy.  It takes all kinds.

Kids eat what is prepared for a meal. Or... they go hungry until the next meal.

Period. Dot.

No snacks, nothing. Eat, or go hungry.

KISS - Keep It Stupid Simple

Regarding hygiene and neat attire.  She bathes and puts on presentable clothes, or.... she does not go with the rest of the family anywhere.  If she is 12, she can stay home alone and starve/stink to her hearts content.

Lock up the pantry and refrigerator leaving only a single plate covered on the counter for her to eat from with a selection. Protein, green veg, etc.

Her chosen hygiene and attire will eventually get her shredded by her peers at school. Something will eventually counter whatever advantage she is getting out of her food, hygiene, and attire choices.

Those who make those choices, have no place in the lives of decent people.

Those otherwise incapable of behaving properly, should be put in an evironment where they can be supported and sternly guided to behave in compliance with established standards.

Unconditional love does not mean unconditional acceptance of unacceptable behavior. Whether or not the perpetrator of that behavior has legitimately Dx'd issues.