Is it necessary to use a go-between or representative to propose?

It is not obligatory, but using a trustworthy intermediary greatly helps protect the dignity of both sides.
An intermediary can convey the intention, carry the biodata, ask sensitive questions and convey a refusal without embarrassing anyone. Khadijah r.a. used an intermediary before her marriage to the Prophet ﷺ, and Umar r.a. offered Hafsah directly to trusted Companions. Choose an intermediary who is honest, keeps confidences and has no vested interest. They must convey information truthfully, without excessive praise or hiding important shortcomings. In Malay custom, the proposal delegation is usually led by respected relatives. See khitbah.