As I’ve started reading more I’ve begun to notice that a lot of words that can be written with kanji are written exclusively with kana, especially with verbs. Is there a rule of thumb for when to use kanji vs kana, or is it just random?
There are several reasons why someone would choose kanji or kana.
Formality. Kanji look more formal.
Grammar. Kana versions are used more often in words that are used abstractly in grammatical constructions.
Style. Some people just prefer the look or flow of one version over the other in their sentence.
Rarity. If a kanji is rare you might just choose to not use it, rather than add furigana.
Kanji often carry some context. Especially if there is very little text, like one sentence on a sign. With pure kana you’d often run into various ambiguities.
Just as an example for what Leebo said, the character Yotsuba in the manga Yotsubato mostly “talks” in Hiragana, because she’s 5 years old or something, and it looks cute, and maybe because the character doesn’t know Kanji yet. Not sure why she sometimes does use Kanji, maybe that cuteness factor isn’t always wanted.
By the way, there are also words that can be written with Kanji, but that form is rather obscure, so they are almost always written with Hiragana. Like 有り難う, which, you guessed it… is ありがとう. I actually saw that word with Kanji for the first time just now.
Similarly, you’ll see a character use hiragana for a loanword that they don’t know the meaning of, as they don’t realize it’s a loanword.
Her speech balloons had some kanji earlier on, but I think that stopped completely in an early volume, as the author/artist decided to use hiragana to show her youth and lack of world experience in her speech.