taru-adjective, adverb taking the 'to' particle
Literary 'taru' adjective describing a joyful or cheerful manner. Often used adverbially with と (欣然と) to mean 'gladly' or 'with delight'. Rare in modern conversation; mainly found in written or formal contexts.
He gladly accepted upon hearing the news.
The old man cheerfully gave directions to the young person.
Also a literary adverbial phrase meaning 'joyfully' or 'gleefully', but 欣然 is more about a calm, willing cheerfulness rather than overt glee.
Standard kanji form for this literary adjective.
Rare variant kanji form; 忻 is an uncommon character.
The kanji 欣 means 'rejoice' or 'take pleasure in', and 然 is a suffix forming adjectives/adverbs describing a state. The reading きんぜん is the on'yomi. The variant 忻然 uses a rare character 忻 with a similar meaning.