忽 is a Chinese hanzi, listed here under the keyword “suddenly”. It is built from 勿 (must not) and 心 (heart). It appears as a component in 5 other characters in this index.
On KumiJi Glyph Explorer, open the parts search and type “must not” + “heart”. 忽 is among the results, together with every other character containing the same shapes.
Open 忽 in the interactive kanji search
to expand every part down to its smallest pieces, add your own names and mnemonic stories,
or search for other characters by their parts.