Plot
- In the manga there are some hints Alicia is seeing someone, even though she never even introduces the guy (?) to anyone.
- It's said several times that the reason she is retiring as an undine is not her marriage, it's the shift in job to working at the Gondola Association, and she doesn't have time to do both jobs at the same time. It's just that doing it all at the same time coordinated well.
- This makes sense when the fact that Alicia explicitly stated she held off Akari's promotion for as long as possible is taken into account. She probably wanted to postpone getting roped into a loveless marriage for as long as possible so that she can cherish more time doing the job she loves as well as being with her beloved pupil.
The tourists could be spirits passing through to their own ultimate destination, in a limbo sort of way. Akari and the other undines received special treatment because they were good people in their previous lives, so they get rewarded by being spirit-guides in the afterlife. The presidents of the various companies are spiritual overseers, disguised as cats. Ai could be an angel, mediating between the two worlds, until she has shown to be worthy enough to be a guide herself.
It could be that Alicia seized Akari's promotion to Prima as a chance to step down as an undine and be with her beloved Akino once more. Alicia likely had some difficulty with her plan once she started to care deeply for Akari as well, which might be the reason the postponed Akari's promotion—but in the end she made the decision she considered best for both herself and her protégé.
Alicia from ARIA is 19 years old and pretty much looks that way. It doesn't gel well though with her supposed extensive experience as an undine, which led to her legendary status. It is also said that she waited a long time before she accepted an apprentice—but if she became a Prima at 16, 3 years is really not that much time. Besides, at 19 she would perhaps be bit young to retire and get married. It all makes sense if her age is actually expressed in Mars years, which means she is closer to 35 in earth time (and in some anime episodes she does look quite a bit older wen she's tired). Of course, at 35 she is still too young to retire, but she merely stops with her hard work as an undine and starts working at the tourist office. It also explains why Alicia is reluctant to get closer to the much younger Akari, despite their emotional connection.
The same holds true for the other Water Faeries, of course—Aqua's atmosphere obviously helps someone to remain youthful for a long period of time. Or it could simply be a case of Generic Cuteness. The only problem with this is the story of Akira and Aika, since Akira had still not promoted when she met Aika, who was about 6 earth years old at the time. That means that Akira (and the other two) would have been about 25 by then in earth years, which doesn't seem quite right.
- Additional problem one with this theory: everyone else on Aqua (including 14-year-old Alice, in 9th grade) counts their ages in Manhome years — this is explicit in the manga, where birthdays are twice a Martian year. Additional problem two: Alicia isn't 19 when she retires, but only when Akari joins the Company — once you add up the series time, Alicia retires at 23 or 24. (Paying attention to ostensible time has its own problems of course, as Alice spends three years in 9th grade.)
- It's still a bit early to retire when you're 23, but perhaps Alicia simply didn't like her job. Also, nobody appears to age within that time period.
- The idea of Alicia not liking her job seems surprising considering the advice that Akino (grandma) apparently gave her, which is to "enjoy everything" (remember near the middle of Animation?) which makes it seem odd she'd retire long after that advice.
- It's still a bit early to retire when you're 23, but perhaps Alicia simply didn't like her job. Also, nobody appears to age within that time period.
- In order to provide tidal pull the terraformers of Aqua dragged a couple other solar system objects into orbit of Mars, maybe Ceres and one of the Jovian moons, or 2 of the Jovian moons, or maybe a couple Kuiper belt objects.
- Based on the last chapter where it's shown that Cait Sith receives Akari's emails, Cait Sith is also probably the one who recommended Akari to Alicia.
- Akari's "e-mails" were shown, right after that page, to actually be blog entries written in the form of letters. "Zenryaku" simply means "Salutations" or something to that effect, but ACS translates it as "Dear Penpal", which causes the confusion. The page right after the one showing Cait Sith sitting in front of a laptop shows three different people reading her letters, implying that they were blog entries.
Impossible crossovers
- This is a little doubtful, as the series implies quite strongly that Aqua (Mars) is a tourist destination for regular visitors from Manhome (Earth), those visitors choose to return to Earth after their visits, and Aqua's population is still small and the Terraforming process not quite complete yet. Plus, there's regular contact between the main character and a little girl on Earth, who not only seems perfectly healthy and hapy, but whose parents are quite happy to return her to Earth with them without so much as suggesting Aria take their child in, since if Earth's in the state the above troper suggests, they'd be pretty irresponsible parents to want to keep their daughter there. Still, it is admittedly implied that Earth is in a pretty polluted, if still liveable state.
- Even if the two worlds don't mesh perfectly, the tone of the two series and the fact that Choro Club did the music to both anime series makes me think there at least ought to be something to this, even if there's not.
- Earth is viable, and humans are still having children, just not enough children so the population is shrinking. But with robots and nigh post-scarcity tech and advanced genetic engineering (cats on Mars, lots of weird things in YKK) Mars got terraformed (extravagantly) and settled. Lots of people on Mars and Earth are deliberately ignorant, leaving them susceptible to wonder at the tricks behind-the-scenes people and robots pull for their entertainment. Though this won't explain the time travel episode.
- Probably they already repaired whatever problems they had on Earth (came home to Manhome) and had even the resources to do some fancy stuff (e.g. turning Mars into a giant Venice knock-off after the original, along with the Netherlands, fell victim to the rising oceans). Bet they made one of the moons of Jupiter into a Netherlands knock-off, complete with windmills, clogs and weed.