Which sentence is correct?

[MsNyree]

Using the word platonic in a sentence.

Sam and I had a good friendship. The relationship started platonically and then became serious.

Should I use platonic or platonically?

[Teia]

Using the word platonic in a sentence.

Sam and I had a good friendship. The relationship started platonically and then became serious.

Should I use platonic or platonically?

Platonically is correctly used. It is an adverb and it modifies the verb started. , OR, you can say :

Sam and I had a good friendship. The relationship started in a platonic way and then became serious.[platonic = adjective]