The Koda Duck is a pet inside Pokemon, in real life it has a round body, a flat beak, and a pair of particularly large eyes, from the overall point of view, the Koda Duck's body is very cute, so the boys will send girls cute Koda Duck.

At the same time, the Koda Duck is a particularly innocent character in Pokemon, and it often commits nerdiness, so a guy sending a girl a Koda Duck may be describing the girl as very nerdy and cute.

Additionally, Koda Duck is a particularly naive character in Pokemon, and will always say things that make people laugh and cry, which is also one of the reasons why boys give girls Koda Ducks.

All in all, a boy giving a girl a Koda Duck means that the girl is very cute, innocent and naive.