Japanese (renzo inc)
Although other apps offer more example phrases and verb forms, Japanese by Renzo Inc. excels in the organization of its contents. It offers a simple, free reference guide for kanji, katakana, and hiragana with tables of diacritics, digraphs, and stroke order animations.
The kanji section divides about 2000 kanji into levels by school grade (which kanji are learned by Grade 1 children, then Grade 2, etc.) and frequency. You can see examples of compound words that use the selected kanji, more complicated kanji that use the selected kanji (a little bit of inception there), and example sentences that use the kanji in context. You can also sort kanji by their radicals, or sort words by parts of speech, adjectives, nouns, verbs, suffixes, expressions, idiomatic phrases, proverbs, dozens of topics, and more.
Although there is a flashcard section, you are probably better off using Kanji Study or Anki, as they offer more interactive flashcard activities and have better formatting.
Japanese is free, but you can pay to have advertisements removed. Another option is Takoboto, which is also free and includes lists of word forms.