What is a kanji letter for FLY?

Fly says TO-BU in Japanese. TO is written in kanji and BU is in the hiragana alphabet.

Do I need a hiragana letter when I use this symbol?

If you want to use it for your design work such as tattoo and art, putting the kanji symbol alone seems cooler and that makes sense! You need a hiragana when using it as a verb.
FYI: The phrase for supporting Karasuno high school in Haikyuu!! is TOBE. This is an imperative form of the verb “fly” and gives strong and determined tones.

How to read
To-BU, HI
Meanings
fly, jump high, move fast
Impressions
It’s a kanji symbol that gives active, free, magnificent, and positive impressions.
Background
The whole shape pictures a bird with its wings flapping.
Free download
You can download the kanji data with transparent PNG format. Each letter is made in 1000px X 1000px. Available for any purposes and no link or credit is needed. Feel free to take it!

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