The Tortus is a Basic pet which specializes in tanking. It has the best base Vitality of the three Basic pets, and also has a good P.atk. This makes the Tortus a good choice for new players who need their pet to absorb some hits from monsters.
There has been speculation of people being allowed to mount an Evo 3 Tortus by sitting on its shell Indian Style. But this has never been implemented.
Though easily overlooked by many because of its tier and other pets serving the same purpose with better stats, the Tortus remains a good choice for first timers.
Granted a Knight/Templar has high enough P.Def that the unity will not matter, a stage 5 tortus with a max "Selfless Defense" will reduce the amount of damage taken by the Knight/Templar by 35%. Best starter pet for a Knight/Templar that cannot yet afford a well staged Gnoll or Stone Golem.
Tortus cards can be purchased from the Merchant NPC.
The build of a Tortus should mainly focus on increasing the pdef and tanking abilities for the pet. The passives should be leveled in order to maximize the tanking ability. The Unity is an essential skill for all classes because of the importance of high pdef for all classes in arriving at dungeon parties. The card and armor skills are also very useful.