Who is Dagobert Briest from Effi Briest and what is their importance?

Dagobert Briest is Effi's witty, young cousin. He is a lieutenant in the Alexander Regiment. Dagobert shows Effi and Mamma around Berlin, and he and later helps find an apartment for Effi and Innstetten. Dagobert is attracted to Effi, and after noticing, her husband asks her if she shares the attraction.