brush, finally, self, relate, follow, here, fast, writing brush radical (no. 129)
聿 centers on a writing brush and the act of writing, which extends to ideas of following a stroke, relating to the self, and finally completing a written line. The radical sense is the same brush shape used as a component.
聿 is a pictograph of a hand holding a writing brush, with the vertical stroke representing the brush handle and the horizontal strokes the bristles. The modern form retains this brush-like shape.
The vertical line is the brush handle, and the horizontal strokes are the bristles. Picture a hand holding this brush, writing a final stroke that relates to the self.
For イチ, imagine the brush making one (一, いち) perfect stroke: 一 -> イチ, and the brush finishes the line.