II, two, second
弐 centers on the number two in formal or legal contexts, used as a replacement for 二 to prevent fraud. It means 'two' or 'second'.
弐 is a formal variant of 二 created by adding 弋 to make the character harder to alter, a common practice for numerals in legal documents.
The top 二 is the number two, and the bottom 弋 looks like a seal or stamp, making it the official, tamper-proof version of two.
For ニ, imagine a knee (ニ) bent under the weight of a formal document stamped with the official numeral two: knee -> ニ.