In Toriel's house, Aliza is drugged by Toriel and starts to hallucinate visually and audibly. Three of her hallucinations are of four SOULs that were unable to escape Toriel's "love". Those SOULs are Perseverance, Integrity, Patience, and Justice. You can tell that it is those traits from the color of their dialogue towards Aliza.
Justice was killed by Toriel in a rather cruel manner. She took the child's asthma inhaler and hid it. The child most likely died from asphyxiation as a result of a likely asthma attack from the dust of the Ruins.
Integrity was killed by Toriel when she poisoned their food. He (I'm assuming it is a he from the shadow in the comic) had asked Toriel how to go home and she poisoned his food, leading to his death.
Patience's death is not explained in the comic. There are no hints on how she (again I'm assuming from the shadow of the comic) was killed. She calls for help and tells Aliza that Toriel said it was unsafe to go outside.
Perseverance is seen in the 14th comic where they ask Aliza to take them home. If the "Pink Box" is Perseverance than it is safe to say the they were stabbed as a result of their escape attempt. They seemed to have had a kind family so why they fell into Ebott, is unknown
There is also a pink dialuoge box in the 15th comic which does not match any previously known traits. The Pink bubble comes from a pair of pink shoes, much like Integrity's ballet shoes. This could, however be Perserverance with a more pink colored speech bubble.