First lets gather all elements. From this answer:
Blood type carries a significant weight in Japanese culture, so much so that Japanese are often surprised when people of other origins are not familiar with their blood type. In their culture, they associate every blood type with a certain personality and demeanor.
Type B
- Independent
- Agressive
- Optimistic
- Friendly and open
- Humble
- Afraid of being alone
- Flexible thinker
- Like to play
What does being a "poster child" means?
The definition of "poster child" has since been expanded to a person of any age whose attributes or behaviour are emblematic of a known cause, movement, circumstance or ideal. Under this usage, the person in question is labeled as an embodiment or archetype. This signifies that the very identity of the subject is synonymous with the associated ideal; or otherwise representative of its most favorable or least favorable aspects.
About the character
Yutaka Hoshino/Peco (星野 裕/ペコ Hoshino Yutaka/Peko)
One of the protagonists and Smile's childhood friend. Peco is loud, cocky, and carefree. He is initially considered one of the best players on the Katase team, but begins questioning himself after a couple of surprising losses. Peco is frequently seen snacking on various junk food items.
So he is saying something in the lines of
- "you could put a picture of Peco in the encyclopedia entry for blood type B's personality."
or - "Peco is a living example of a blood type B's personality."
