As a note, as this site has been the site to which many important events have occured, this article is laden with spoilers. Read ahead at your own risk.
That tree on the hill in the summer of 1998.
The tree on the hill behind Hakuryou that overlooks Hiiragi Town. Shows up everywhere. First made its debut in Kimi ga Nozomu Eien, being a location held by the students that "if a couple formed beneath that tree they'd be together forever" or something to that effect.
Kimi ga Nozomu Eien/Akane Maniax Edit
Goes on to appear several more times in KimiNozo, such as the site of Haruka confessing to Takayuki.
Suzumiya Haruka leaning on that tree as she confesses to Narumi Takayuki.
As well as when the 4 friends take a photo to commemorate the summer.
Depending on how history flows, that tree can make many of several possible appearances, such as here, when after graduating in 1998, the couple of Haruka and Takayuki meet up with their close friends Mitsuki who is pursuing her dream as an olympic swimmer and Shinji who's not really doing anything in 2001.
After decades of putting up with high-school drama under your branches... you'd be tempted to summon Space-Bugs to destroy all of humanity too.
After its various appearances in Kimi ga Nozomu Eien, that tree also appeared in Akane Maniax.
As the site where Suzumiya Akane tells Gohda Jouji about her past as how she'd like to remain as friends with Gohda.
Also the site where Kagami Sumika gives Gohda a handkerchief.
Moments before fulfilling his destiny and becoming Tekkumen, Gohda Jouji paid one last visit to that tree on the hill.
The tree also shows up many times throughout the course of Muv-Luv.
It shows up a great many times in Extra, such as the site where Takeru must choose between Meiya and Sumika.
And as the site where Takeru confesses to Sumika.
It appears several other times in Extra as well, such as the site where Takeru also reveals his capabilities as a Tsundere.
As well as the site on which Takeru and Sumika end their Christmas on.
Almost naturally, that tree also shows up in Sumika's Afterstory in Muv-Luv Supplement, Sakura no Hana ga Saku Mae ni.
And again as Takeru presents a new "Santa Rabbit" to Sumika.
It also appears as the site where Takeru and Sumika make up in Sakura no Hana ga Saku Mae ni.
That tree also appears multiple times in Alternative.
Such as when Takeru takes Yashiro Kasumi to various places while bonding with her. If you look closely you can make out the base of that tree.
And it's also the location where Tsukuyomi Mana of the Japanese Imperial Guard asks a favor of Takeru.
Among other things, it the site at which Meiya tries to encourage Takeru after the 00 Unit tries to push him away.
It's also the site the 00 Unit tries push away Takeru, by projecting a rather graphic memory onto him, unfortunately for the 00 Unit, Takeru had long since turned his mind to steel and became a superhero, and kept up with himself, such that inferior grade projections which lose one rank could not faze him.
Kimi ga Ita Kisetsu (2011 remake) Edit
During early development, the tree also seemed set to appear in the 2011 remake of Kimi ga Ita Kisetsu, despite none of the Isumi sisters being Hakuryou students, especially not Marika, who is seen near it.
However, it was made clear by the game when it finally came out and was completed by patches that only characters that made sense to be by it appeared by it.
What appears to be that tree,
Maejima Masaki, and Isumi Marika.
That tree making its debut.