It depends. If you're reading manga on legitimate websites that have the rights to distribute it, it's not illegal. However, if you're getting it from illegal torrent sites or unlicensed platforms, then yes, it's against the law.
The legality of reading manga online depends on the source. Legitimate platforms with proper authorization make it legal. But most random sites offering free manga are likely illegal and can get you in trouble.
It's not mandatory to read the Pokemon manga in order. However, doing so can provide a deeper appreciation of character development and plot progression. Skipping around might cause a bit of confusion at times.
You can start by familiarizing yourself with the characters and the basic plot. Then, take your time to soak in the details of the art and the story progression.
Well, sometimes you can find Pokemon manga on bookstore websites like Amazon or Barnes & Noble. Another option is to search for dedicated manga platforms that specifically focus on providing a wide selection of manga titles, including Pokemon.
In the Pokemon Alola Ultra Adventures fanfic regarding Greninja, it may start with Greninja arriving in Alola. It could then get into battles with the island Kahunas' Pokemon. Since Greninja is a very powerful Pokemon, it might be a key figure in some important events in Alola. For example, it could help protect the Alola region from some kind of threat that is not common in other regions. And it may also have some interactions with the Alolan forms of known Pokemon, which would be really interesting to read about.