Daihitsu no Koi: Ojou-sama no Furi o Shite Kon'yakusha ni Tegami o Kaiteimasu

Kate is a maid who writes letters on behalf of her master Mary to her fiancé, Elliot. Although she knows she shouldn't, she finds herself becoming more and more attracted to Elliott's personality that she senses in his letters. She tries to keep these feelings hidden, but then Elliott decides to come visit Mary in person for the first time in several years…

(Source: Kodansha, translated)