noun
convert; person who has reformed
Refers to someone who has undergone a change of heart, especially in a religious or moral sense. Often used in contexts of repentance or conversion.
彼は改心者として教会で証しをした。
He gave his testimony at church as a convert.
かつての犯罪者が改心者となり、更生を語った。
The former criminal became a reformed person and spoke about rehabilitation.
回心 is a near-synonym often used specifically for religious conversion, while 改心 can also imply moral reformation.
Compound of 改心 (reform, change of heart) and 者 (person). The exact historical derivation is uncertain, but the word follows a common pattern for agent nouns.