Final Fantasy X’s story is symbolic of a lot of things. Yet the characters tend to be metaphoric to the celestial bodies. Several allusions exist to both the sun and the moon in, Tidus and Yuna.
First, take a look at the etymology of the world’s name:
The name Spira refers to the word “spiral”, alluding to one of the main themes of Final Fantasy X. The word “spiral” has multiple meanings depending on context. In geometry, it is a curve acting as the focus of a point rotating around a fixed point that continuously extends from that point. Informally, it can be used to refer to a helix. It is also used to refer to a high-momentum, self-sustaining process that can’t be stopped or accelerated instantly.
All of these terms would also suggest a cycle. The phases of the moon orbit the Earth often times in front of the sun. Spira is the land where the representations of the sun and moon take place.
Now take a look at our main protagonists, Tidus and Yuna. Tidus’ name is based on:
(t)he Okinawan word tida means “sun”. The sun theme is reflected in the crest and sigil needed to power up his Celestial Weapon, and possibly in his primarily yellow outfit and blond hair.
This is clear enough. Equally clear is Yuna:
And according to Wikipedia:
Nomura also said that he adorned Yuna’s dress and necklace with images of the hibiscus flower also called “yuna,” and that her name carries the meaning of “night” (夕な) in Okinawan, establishing a contrast between her and the lead male protagonist of Final Fantasy X, Tidus, whose Japanese name (ティーダ) translates to “sun” (太陽) in Okinawan.
Also consider that the sun and moon connotations carry over in other symbols, such as Yin and Yang, whereas:
Yin is the negative/passive/female principle in nature, and may allude to many other concepts e.g. the moon…
Yang is the positive/active/male principle in nature, and may allude to many other concepts e.g. the sun…
Tidus and Yuna both end up crossing paths through their respective “pilgrimages” (like an orbit). Tidus, the Sun, brightens Yuna’s day. In Final Fantasy X-2, she’s more active – a “reflection” of Tidus.
When Yuna finally completes her pilgrimage, Tidus disappears. Actually, Yuna is the one moving away from Tidus, as he’s disappearing from her sight.
It’s a happy ending though, as they reunite.