F-Rank Shoukanshi, Pet Atsukaide Kawaigatteita Shoukanju ga Bahamut ni Seichou shita node Bouken wo Yamete Saikyou no Ryuu Kishi ni naru

From the weakest summoner to the world's greatest dragon knight! Adventurer Jay was ridiculed by those around him as the "Lizard Summoner." He was looked down upon because the only summonable beasts he could summon were small lizards. Because of this, Jay was unable to form a party with anyone, and continued his lonely adventures while doting on Kyuk... But one day, Kyuk suddenly grew into a gigantic, majestic dragon - Bahamut! Jay took this opportunity to start working as a courier, and before long, word of the "Courier Dragon Knight" spread throughout the royal capital, and everyone began relying on him! From a mocked F-rank summoner to the world's greatest dragon knight - the unrivaled fantasy life begins!!

(Source: Shueisha, translated)