Otonari ni Ginga

Ever since their father died, Ichirou Kuga has struggled to support his two younger siblings on nothing but a small inheritance and his passion for drawing manga. But it's becoming harder to keep up with his growing responsibilities and deadlines, especially after his last two assistants quit to follow their dreams. Just as he's nearing his breaking point, the beautiful and scarily competent Shiori Goshiki applies to become his new assistant. But there's something almost otherworldly about Goshiki, and soon Kuga finds his reality turned upside down when she suddenly declares them engaged to marry!

(Source: Kodansha USA)