"...I would say that spending time with you feels like a miracle" | Dsiclaimer: ORIGINAL NOT BY ME, ITS A MODIFICATION

Why Yume is alive and suddenly flies a F-15EX ? No, its just a AI version of Yume that was implemented in the F-15 .

Kuchinashi Yume

Little is currently known about the final president of Abydos High School's old student council, other than that she was "naive" and a "fool" said by the Former Vice President Takanashi Hoshino.

Despite her position as the student council president, her competence wasn't on par with her title, approaching major problems recklessly without being aware of the consequences her actions might cause.

A student who believed in miracles. A bit too much according to Takanashi Hoshino. Although she had a kind attitude, she was deemed irresponsible and reckless by her own junior. A fitting way to describe her is that she was blinded by her own hope. Despite everything she went through though, a smile as bright as her hope can always be seen on her face.

Yume has a cyan long hair reaching her knees, and long cowlick, reaching her neck. She has golden yellow eyes. Band aids are scattered around on to her arms, and legs.
~Blue Archive Wiki

Story

A F-15EX Eagle II was modified and implemented a AI of Yume requested by Hoshino. The plane is a result of a collaboration with the Millennium Academy. Although the plane is controlled by an AI but the students from Millennium tested and the results were great, proofing the AI controlled aircraft to be efficient in combat. Funny thing is, the second seat is for the second pilot to control the aircraft if the AI has a system malfunction.
~Self written story

F-15EX's irl information

The McDonnell Douglas F-15 Eagle is an American twin-engine, all-weather fighter aircraft designed by McDonnell Douglas (now part of Boeing). Following reviews of proposals, the United States Air Force (USAF) selected McDonnell Douglas's design in 1969 to meet the service's need for a dedicated air superiority fighter. The Eagle took its maiden flight in July 1972, and entered service in 1976. It is among the most successful modern fighters, with over 100 victories and no losses in aerial combat, with the majority of the kills by the Israeli Air Force.

The Eagle has been exported to many countries, including Israel, Japan, and Saudi Arabia. Although the F-15 was originally envisioned as a pure air superiority fighter, its design included a secondary ground-attack capability that was largely unused. It proved flexible enough that an improved all-weather strike derivative, the F-15E Strike Eagle , was later developed, entered service in 1989 and has been exported to several nations. Several additional F-15 variants have been produced.

The USAF had planned to replace all of its air superiority F-15s with the F-22 Raptor by the 2010s, but the severely reduced F-22 procurement forced the service to operate some F-15C/Ds until 2026 and replace them with newly built F-15EX Eagle II in order to retain an adequate number of air superiority fighters. The F-15E Strike Eagle is expected to continue operating in the USAF into the 2030s. The F-15 is in service with numerous countries, with production of enhanced variants ongoing.