grass
卉 centers on grass as a general category of low-growing plants. It is a somewhat literary or collective term for grass rather than a single blade.
卉 is a pictograph of three grass blades growing upward, later stylized into the modern form with 十 above 廾. The character represents grass collectively.
The top 十 looks like a cross-shaped tuft of grass, and the bottom 廾 suggests two hands holding or tending the grass. Together they form the idea of grass as a plant.
For キ, imagine a key made of woven grass: key -> キ, and the grass key unlocks a meadow.