In the Sandman series by Neil Gaiman, the character designs for Morpheus and Death are similar enough to show the sibling relationship between the two, however the Lord of Dreams is the more relatable of the two characters despite his less human character design. This could potentially go back to Scott McCloud's ideas regarding identification through simplification. Personally, I enjoy the slight humor behind Death being the more human of the two characters.
Lets take this one step deeper though. Morpheus' character design actually shifts depending on where he is and the other characters present in the scene. For example, when in the human world with his sister Death he appears slightly more solid and less ephemeral as shown below. Also shown is a scene where he threatens one of the characters and his design has changed to be more intimidating. The main changes are things like his eyes turning red and his clothing shifting as well. The dark shadows around his eyes create a more harsh contrast and create a stark visage for the character, as though he were the bogeyman from a child's nightmare.
In my opinion, this ephemerality of Dream is what makes him a much more relatable character than Death, despite her appearing more human. He can change to represent the situation and the feelings he wishes to evoke in other characters and the feelings the creators want to evoke in the readers. That being said, this ability to change designs slightly allows Dream to become a physical representation of the feelings a reader experiences throughout the plot of Sandman. It becomes easier overall to feel angry when we can see the rage in how his character is designed as a whole, as opposed to strictly a facial expression. Likewise, it is easier to feel compassion for Morpheus when the reader can see his whole face during an emotional conversation. The ultimate result of his changeability is, therefore, what allows him to be such a relatable character, however, we relate more on a base emotional level since his design begins to represent different emotions the reader feels.
All in all, it is a brilliant effect achieved through very simple means, while still keeping the character consistent with certain features like his hair, skin tone and the sharp planes used to design Dream's face.