A couple alternatives to WaniKani are:

* https://kanji.garden/

* https://www.readthekanji.com (includes hiragana and katakana as well)

You can also use the following for just memorizing hiragana and katakana:

* https://djtguide.neocities.org/kana/

* https://www.kana.pro/

And then there are online resources like the following that provide a guided, DIY approach to learning Japanese as a whole:

* https://www.tofugu.com/learn-japanese/

* https://guidetojapanese.org/learn/

* https://docs.google.com/document/d/10bRzVblKVOsQJjTc2PIi1Gbj...

It has a built-in spaced-repetition system with predefined, extremely silly (but effective) mnemonics to help memorize the various kanji (and their various readings) and vocabulary.

The first three levels are free, with the prerequisite of learning a bit of hiragana and how to type them on their website (I don't know how well this translates to an actual Japanese keyboard). The emphasis is on reading, and they suggest you don't put a lot of effort into understanding actual Japanese in the wild (or media) until level 10, when you have enough vocabulary to not be completely lost.

I'm only on level 5, just trying to be somewhat less functionally illiterate for an upcoming Japan trip than I was when visiting Taiwan 20 years ago. Although I can't read many kanji, I'm often able to recognize a few as well as enough of the radicals of the ones I don't know to sometimes make sense of them.

I hope this isn't too nit-picky, but I'd qualify that it's great (fantastic, even) for memorizing kanji and their readings, but not for reading and understanding Japanese text. You need something like a grammar course and lots of comprehensible input for that.