Naoki Urasawa is a pretty sneaky writer, the more you read Happy! the more you realize tennis is just a plot device for him to insert the psychological elements which he is most known for. If you are thinking Happy! will have a lot of technical shounen-esque tennis content, you are reading the wrong manga. A lot of Happy! is about dealing with human relationships. I would say this writing strategy becomes very repetitive as almost every arc is about Umino's enemy setting her up to get the reader's blood boiling because Umino seems to be always oblivious about it.
Now for the characters, besides Sakurada, it seems like all the antagonists in this manga are heartless and one-dimensional. They all have the same mindset that they think Umino is a fake, got lucky in al her wins, and they all want to ruin her life. The same could also be said for Umino's rivals with a few exceptions. Oh and as far as the sport rivalry dynamic is concerned, there is very very very little if any. It is really about Umino overcoming life hurdles thrown at her, so there isn't gonna be any training camps or that rubbish you see in every shounen manga.
The art is pretty good, very easy to interpret the panels and very easy to tell which character is an antagonist and who isn't. The games are also drawn really well, no flashy crazy name techinques or exaggerated movements. Just simple run back and forth and different strikes.
I would have to say the only thing I did not like about Happy! is the ending, it felt like there should've been more to say and I didn't like the result of the last game, it felt very forced.
