one (in documents)
壱 centers on the numeral one, used specifically in formal or legal documents to prevent forgery, similar to writing 'one' as 'one' instead of '1'.
壱 is the modern simplified form of 壹, which originally depicted a gourd or vessel and was borrowed phonetically to represent the number one in formal contexts. The modern form retains the top 士 and simplifies the lower part.
The top 士 looks like a formal seal, and the lower part 冖 and 匕 resemble a container holding a single item. Picture a sealed document with one official stamp: 壱 means one in documents.
For イチ, imagine an itchy seal on a formal document: itchy -> イチ, and the seal marks the one and only official copy.