Terra's name means earth and his basic elements are earth and strength. He is known as the vast earth protecting bonds. I've already covered the idea that everything he does is for the protection of his friends and his loved ones. He holds his bonds of friendship with Aqua and Ventus in such high regard that he would fight and die for them. He would overcome all obstacles and protect them with every ounce of his final and failing strength. He protects Ventus from being killed by the person he respected most. Even without a physical body he manifests his armor and uses his keyblade to protect Aqua. Despite differing opinions and his internal struggle, everything he does is for Aqua and Ventus.
The earth itself is considered a steady force. Most of the symbolism is for things below the earth or close to ground. I think this is one reason that Terra usually battles so close to the ground. In fact, it could have to do with his height and weight, but it's an interesting thought to think that he somehow draws power from his feet being on the earth too. This could be why he doesn't get the amazingly helpful double jump and why his movements are slower and more bulky.
Water erodes earth; it is believed that the earth will always succumb to it. I believe this is why the things that Aqua (water) says and does hurt him the most. When she watches him or questions him, he really takes it to heart. Her doubt weighs on him and makes his heart heavy and creates more insecurity. This is interesting to me because it always seems like Aqua's words are the ones that cut Terra deepest, even when she's showing concern.
One particular point I liked was that earth also represents community and belonging. Terra creates bonds of friendship where ever he goes. But his driving goal is to master his darkness and return home. His driving force is mastering the darkness so that he can return home, maybe so he can belong there again. Earth is also considered a unifying or connecting force. Terra's actions in other worlds have direct effects on how Aqua and Ventus react. The idea that Terra stole Aurora's heart bothers them. The fact that Stitch makes his own wayfinder to look like Terra's inspires them. At the end of the day, saving Terra meant everything to Ven and Aqua. At the end of the day, all of Terra's mistakes and choices led to his own defeat and possession but did not stop him from protecting or looking after his friends.
The most familiar representation is probably earth nourishment, stability, permanence, security and solidness. I would say that through Terra's choices ultimately Ven lives. As far as Ven was concerned if Terra was on the job, everything would be okay. This gave him security. I also believe that his interactions with Riku lead to nourishment (in Riku's case the growth of desire to leave his home world and the power to do what he needed to). Even if he couldn't master the darkness himself, his successor does. Terra is a testament to solidness in and of itself. He fought with the darkness and though he struggled with it, he was only ever calm and composed when addressing his friends. He was only ever to the point and unyielding in his decisions. This is why he fights Eraqus despite knowing that it is wrong to fight his master. His desire to protect was much stronger. His own growth may have been stunted by his insecurity but his successor masters the darkness.
Over all, I would say that Terra's steady personality and his strength are most earth like. Everything he does is for the growth and protection of his friends. He protects those bonds with his life. Beyond his life, beyond his body, and he would always make the choice to do it. Darkness in his heart, weakness abound, the only thing that matters is protecting those he loves.