Japanese uses two phonetic writing systems, hiragana and katakana, which each consist of 46 characters, one for each mora (similar to a syllable). These are derived from Chinese characters used for their sound value, called man'yōgana.
For a given mora, some hiragana and katakana are cognate, deriving from the same Chinese character, while others derive from different Chinese characters, due to use of different Chinese characters to represent the same Japanese sound. These variant forms, called hentaigana, were used until 1900, when their use was eliminated in a spelling reform.
Kana deriving from the same character may look very similar, while in other cases they can be quite distinct, usually due to different style of writing, though sometimes because a different component was used (as in re: れ レ from 礼).
These similarities will generally not be a cause of significant confusion, due to their having the same pronunciation and different styles, but may be a useful mnemonic.
Of the 46 morae in modern Japanese kana, most (31, 67%) are derived from the same man'yōgana in both hiragana and katakana, while the remainder (15, 33%) are derived from different man'yōgana.
Note also that until the 1900 spelling reform, む and ん were used interchangeably, usually to indicate mu, but sometimes (ambiguously) to indicate syllabic n. Following the spelling reform, mu was restricted to be spelled む, and ん unambiguously indicated syllabic n, which is the current use.
The below are listed: hiragana, katakana (man'yōgana), with non-cognates indented so as to stand out.
In some cases this yields very similar characters; subjectively:
Quite similar: