So before I start critiquing anything, I do want to make note that I am not a fan of bases and will, given the chance to do so, always encourage people to not use bases at all. Normally I would prefer that bias to not show up in these critiques (which is why I make mention of it) but I'm afraid I don't think that's going to be possible here and there are some underlying anatomical problems with the base you've chosen and I feel the need to make note of them.
That said, here's what I can offer:
Firstly, I'm not entirely sure what picture the original base was traced from (as the link in the description of the piece no longer works) but there are some fundamental problems with the anatomy of the two figures--even when taking into consideration the anime style of their design. The larger character's right shoulder, for example, [image left] is protruding too far out from his body. The way he's holding the younger character, his arms should be closer to his body to better support the weight. Additionally, there are perspective problems with the child's arm as, with how the larger figure is angled, that arm should probably be somewhat nearing the audience, which isn't evident here. I point is these problems out because if you choose to continue with bases (which I personally advise against but hey, it's your prerogative) then you should keep an eye out for and shy away from bases with anatomical discrepancies such as this one.
Onto the lineart. I'm not entirely sure what program you've used to do this piece, but I'm confused as to why you would trace over the base lineart (considering bases are usually made so... y'know... the base image is already there to be customized). The newly added lines are visibly shaky and thicker than the original lineart for the base, which is very evident when looking at the two characters' eyebrows, mouths, and the flower gift. I would suggest trying to find a way to keep that lineart steady and to change the size of the lineart itself to make the piece consistent. Thicker and thinner lines can work together in tandem, but there typically needs to be a reason for the difference in thickness or quality of the lines and there simply doesn't seem to be a reason for that here.
With regards to the clothing, the collar of the purple shirt is noticeably high along the blue character's back. Shirts tend to rest predominantly on the shoulders so the place where the neck tapers off into the shoulder would be the most traditional place to put the collar. You've seem to put it higher here and that tends to only work on high-collar shirts, which require a different kind of composition and weight distribution (namely that the collar tends to either cling entirely around the neck, or is simply supported upwards by the shape of the fabric).
Additionally, the hair of the characters is far too low down on their faces. There should be a good bit of visible skin between their eyebrows and their hairlines (i.e. the forehead) but that space seems to be entirely missing here. The hairline even seems to cover over the actual eyebrows of the characters. I understand the sort of hairstyles you were going for but I'm afraid they're placed too low on the characters' heads.
Finally, there's a lot of inconsistencies with the placement of the shadows. The blue character sports shadows on the side of their face and one side of their hair, indicating that the light source is coming from the left side of the image, but if that's the case, there should be shadows on the back of their neck. The younger character they're holding, as well, doesn't seem to be casting a shadow on the blue character at all, which is confusing based on the implied light source. I understand that these are shadows implemented based on the original base you used but these shadows are lacking. There should be shadows between the two characters where they connect, there shouldn't be a shadow on the blue character's wrist (as that part of the body would be exposed to light) and the blue characters fingers on that hand should probably be in shadow since, realistically, they would curl around the smaller character. In having added hair and fabrics to the characters, I would have suggested adding additional shadows and highlights to match those additions, rather than adhering to the pre-made shadows present on the base.
Overall, a steadier distribution of the lineart and a lot more attention to details and anatomical consistencies will definitely help your work.
That's all the critique I can offer up at the moment.
Hope it was helpful~