Usually, the Kohein Gadol is not really seen as a "regular" person, whatever that is, but rather as a symbol of the holiest of those who are part of MAMLECHET KOHANIM V'GOI KADOSH, a kingdom of kohanim and a holy nation.
In this week's sedra, Aharon takes center stage, even with Moshe Rabeinu stepping back to let his older brother bask in that glory.
The garments of the kohanim in general and the Kohein Gadol in particular are L'CHAVOD ULTIFERET, for honor and glory. Later in the Torah, when the Torah describes the Kohein Gadol, it is with such exalted terms that we have a problem thinking of him as "one of us". And, as great as Aharon was, he should be seen as one of the people.
That's hard to do, especially with the Torah's account of the SEDER AVODA of Yom Kippur in the Beit HaMikdash (or the Mishkan, as the case may be). And even more especially, the fact that only the Kohein Gadol entered the Kodesh Kodashim on Yom Kippur, facing death from heaven if his KAVANOT, intentions and actions were not perfect.
There is a small detail in the treatment of the Rambam of the topic of IBUR HASHANA (intercalating the year by adding another Adar) that puts a real human face on the Kohein Gadol. But let's back up for a few moments and introduce the topic of IBUR HASHANA.
In our fixed calendar, the one we use when we have no Sanhedrin to shape the calendar otherwise, we have a set pattern of years that have two Adars. Specifically, seven years of every 19-year cycle are 13-month years. More specifically, years 3, 6, 8, 11, 14, 17, 19 of each Machzor Katan (as it is called, to distinguish it from the 28-year cycle of the Sun) have two Adars. This is a rigid pattern without our ability to exercise any discretion.
With a Sanhedrin, the procedure is different and somewhat flexible. A committee of 3 to 7 members is set up for the purpose of deciding whether a given year will be intercalated to produce a SHANA M'UBERET.
This committee first determines by known calculations of astronomy, whether Pesach will begin before or after the upcoming vernal equinox. If before, then they must declare the year M'UBERET, by announcing that the month following Adar will be another Adar, and not Nisan. This declaration should be made in Adar, but may be made as early as right after Rosh HaShana.
Declaring the coming year M'uberet has no validity; only THIS YEAR… If Pesach will begin after T'kufat Nisan (our name for the vernal equinox), then they can leave the year with one Adar, or they can still intercalate it based on other factors. Has the barley begun to ripen. Have fruit trees begun to blossom and bud. How hard a winter has there been and how long will it take to get ready for Pesach in Yerushalayim. Other factors can influence the decision.
Inter alia, Rambam states that a Kohein Gadol could not serve on the panel to decide one or two Adars. Why not? Because the KG might think to himself – if I voter for another Adar, then the coming Yom Kippur will be in the chillier weather of October, rather than balmy September. And I immerse in a mikveh five times on Yom Kippur. It will be more pleasant for me to oppose the extra month. Not an acceptable factor in the determination.
Do you mean to say that THE Kohein Gadol, that holy and spiritual paradigm will let something so… so mundane influence his vote? Apparently, YES. And do you know why? Because the Kohein Gadol was/is human. That's not a fault – it makes him one of us and it should increase our respect and honor for him. He is not a cardboard cutout; he is a real person. Now we can relate to him on more significant levels. Imagine how Aharon the person felt when two of his sons fell dead. Imagine his feelings as Yom Kippur approaches. And much more.