river willow
杞 refers specifically to a type of willow tree that grows near rivers, the river willow.
杞 combines 木 (tree) with 己, which likely contributes the sound, to name a specific tree, the river willow.
The tree 木 on the left and the winding shape 己 on the right suggest a willow tree with drooping, winding branches by the river.
For コ, imagine a small child (子, こ) sitting under the river willow, playing by the water: 子 -> コ, and the tree shelters the child.
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