Ling Yao is not your average 15 year old boy. He's the twelfth son of Xing's Emperor and is currently in Amestris trying to find the "Way of Inmortality". Contrary to Roy, there's a certain taste of selfishness to his actions. His reasons are basicaly increasing his chances to be the future Emperor's heir, which would improve the Yao Clan's position.

As time passes, however, we begin to notice how Ling's motives may not be as self-centered as they seem. When encountering Fuhrer King Bradley, he delivers a speech which indicates he's probably more idealistic that one would have thought at the beginning. This makes Ling a kind of Xing-esque Roy, in the sense that what they're both trying to attain is similar.

     

Their meeting brings some hope for both their countries, and the chance to improve their current situations. The instant respect both men feel towards the other is only the logical consequence to the incredible help they've lend each other already, even before their actual meeting.

Ling: You people... I'm not too clear on the details, but you're aiming for something bigger than a homunculus or the Philosopher's Stone, right?

Despite Ling's childish appearance, he's got quite a high level of maturity for a boy his age, probably due to his position in his country. After all, if the current Emperor were to die, there's the possibility he could be designated as leader of Xing, even if he's only 15. Either way, it's obvious Ling considers Roy a valuable ally -- and that he returns the feeling and won't fail to colaborate with the boy in the future.

However, Roy is still unaware of the events that followed their separation in the forest -- that in fact Ling is no longer quite himself, but the vessel in which resides a reborn Greed.