two
貳 centers on the number two, often used in formal or legal contexts to prevent forgery, similar to how 壱 is used for one.
貳 is the traditional form of 弐, combining 貝 (shell, money) with 弍 (a variant of 二, two). The shell component likely relates to formal financial documents where complex numerals were used to prevent alteration.
The top part 弍 looks like a stylized two, and the bottom 貝 (shell, money) suggests a formal, tamper-proof way of writing 'two' on important documents.
For ニ, picture two shells (貝) side by side: the word 'knee' sounds like ニ, and you need two knees to stand. Imagine balancing two shells on your knees to remember the reading.