We know that Harry is the middle child, and he’s surrounded by two sisters. I headcanon that, from a young age, Harry has been able to braid hair.
And not just the standard braid. His sisters would nag him to do it so much that he eventually learned all the complex styles (French, Dutch, fishtail, pigtails, braided crowns, braided buns, etc). The more he did it, the better he got at it. As he gets older and starts to steal from Lady Tremaine’s shop, he picks up a few skills just from watching Dizzy and her relatives work. I mean, if he can play the piano (another headcanon), he’s gotta have some good fingers.
Now imagine a much younger Harry showing up to the Chip Shoppe in search of his sea witch friend only to find the place closed. Being the juvenile delinquent he is, he sneaks in any way and quietly makes his way upstairs to Uma’s room. All he can hear is a symphony of curses and swears only a sailor should know (and he, as the son of Captain Hook, would know what that sounds like). He peeps inside to find Uma standing in from of a mirror at her vanity with her hair down and damp from a shower. She seemed very angry and distraught over something. What he didn’t know was Mal had just dumped a bucket of shrimp on her head earlier that day, and she couldn’t get the smell out no matter how many times she washed it.
She sat down in the chair before her vanity and threw her face into her hands with a frustrated grunt. Harry was especially baffled when he heard her start to sob. Still young and innocent, Uma was shattered not only by the sickening smell of her hair but also the fact that her best friend and partner-in-petty-crime just betrayed her. Now she knew loyalty didn’t mean a thing on the Isle.
Little did she know, Harry had silently moved into her room. Since he didn’t know any better, he assumed she was just upset about her hairstyle or something along those lines, and he had a skill set that could fix it. He wasn’t really bothered by the smell either; it smelled like the ocean, which he through was great. Uma didn’t notice his presence until she felt his experienced fingers tenderly brush against her hairline. She was originally startled by the contact, but when she locked eyes with Harry in the mirror, she let him proceed without any protest. Her hair wasn’t as long as it is now, but Harry still took his time to make sure her braid was perfect. When he was done, Uma looked at herself in the mirror with complete awe. He braided a thin crown around her haid and pulled the remaining hair into a loose but classy fishtail braid.
“There ye go, yer hair’s all nice,” Harry said when he was done, “Now can ye stop cryin’? It’s freakin’ me out.”
And thus Uma was reminded of who her real friend was.