• 字じ — character, letter
• 名字みょうじ — surname
• 漢字かんじ — Chinese character
• 数字すうじ — number character
• 文字もじ — written character
• 赤字あかじ — deficit (lit. red letter)
• 活字かつじ — printed type
• 字幕じまく — subtitles / captions

“字” is made from 宀 (roof) over 子 (child), picturing a child under a roof — an early image of a child learning at home. This kanji later came to mean a “written character,” and is central to words related to writing systems such as 漢字, 文字, and 数字.

この字じは 読よめますか？
Can you read this character?

赤字あかじに なってしまいました。
We ended up in the red (deficit).