Tsuihou Sareta Koushaku Reijou to Iku Boukensha Seikatsu

Erica Solunzari, the daughter of a Marquis, was caught up in a conspiracy and exiled from the country under the pretense of "eloping due to a forbidden romance across social classes."
Shin Longdagger, the second son of a poor Viscount family who was chosen to play the role of her lover, happily accepted the offer, even though he fully understood that doing so meant the end of his life as a noble.
The reason? Because Erica was Shin's first love!!