Shaman King is, in many ways, a groundbreaking and expectation-shattering anime and manga series. Though it was on paper on a fairly typical shonen, it went against the grain by having an empathetic message that opposed needless fighting and conflict. Another way in which it went away from the norm was how it portrayed its resident main couple.
Yoh and Anna are two of the series' main characters, but their fierce and sometimes opposing ambitions -- or lack thereof in Yoh's case -- betray them being an engaged couple. This actually makes both characters stronger and especially makes Anna an incredibly interesting female character.
Yoh and Anna Are Total Opposites
Yoh himself is quite different from other shonen heroes in that he's so laidback. This attitude comes off almost as laziness, as the young shaman is much more interested in gazing at the stars and absorbing his surroundings than he is in pursuing much of anything else. Even his supposed goal of becoming the Shaman King takes a backseat to just chilling and abstaining from the rat race of life.
Anna, on the other hand, is the total opposite of him, especially considering their being in an arranged marriage at such a young age. Anna is determined to become the Shaman Queen, and once the Shaman Fight officially begins, she becomes obsessed with whipping Yoh into shape to claim the crown. Thus, her already blunt and harsh demeanor became even more so toward her future husband, hoping to snap him out of his slacker ways and get him to knuckle down and prepare for the coming challenges.
To be fair, none of her seemingly harsh treatment of Yoh is unwarranted. When the two reunited in Yoh's hospital room, he had recently sustained grievous wounds from a fight with Tao Ren. Her training regimen promptly begins afterward, with Yoh's life being upended as he fails to get a moment's rest from his overbearing and authoritative girlfriend.
Why Yoh and Anna Are Such a Great Couple
Yoh and Anna's relationship is very much the reverse of most shonen romances. In the case of characters like Naruto/Hinata or Sasuke/Sakura, the female member of the pairing swoons or looks up to the guy in an almost one-sided manner. Sure, Anna admires Yoh in her own way, but she's definitely the more proactive force in the relationship, mostly due to Yoh's aforementioned laziness. Whereas other female characters' strengths are usually in spite of something else, Anna's strong will and equally as effective powers make her pivotal to the series' fights and progression. In fact, it's her actions that sometimes turn the tide of a battle, usually in Yoh's favor.
Likewise, Yoh is the one in fear of or constantly set back in some way by Anna's actions, as opposed to Anna being the reactive partner. Again, this makes her much more active than say, Chi-Chi, who is many times simply a bystander responding to Goku's less-than-satisfactory performance as a husband and father. While Chi-Chi's own reactions to this may occasionally put her in opposition to Goku, she doesn't push him in the same way that Anna does. Anna's attitude even sees her standing up to series villain Hao, who actually enjoys her company -- sometimes more than Yoh does.
At the same time, the two do develop a strong bond with each other, with Anna admitting later on how much she loves Yoh. This of course sees the two finally get married and have a son in later stories. This means that the two still have an actual relationship and affection for each other, and despite it all, they seem to actually show romantic attention more than many other shonen couples. Unfortunately for the slacker Yoh, however, this is usually interlaced with tons of actual effort to become the Shaman King.