「カブのイサキ」より、シロさん。　シロさん・白さん・ハクさん　ツマンネ。

Shiro from "Kabu no Isaki".
What follows in the original is a brief explanation for the image that's much quicker in Japanese than in English, but it can be summarized thusly:
1. Shiro's name is normally written in katakana (シロ).
2. Among the many possible readings of the kanji 白, two of the more common are "shiro" and "haku".
3. And since "Shiro" is also "Haku", there you have it.

Technically, the "Haku" in Haku's name uses a different kanji, but that never stopped a good pun.