"...I'm not cold anymore... ...This feels amazing!" - Zack Kelvin
| Zack Kelvin|
|Riddle School 1|
|Riddle School 2|
|Riddle School 3|
|Riddle School 4|
|Favorite Thing||Anything not cold|
|Least Favorite Thing||Freezing into ice|
Zack's redeeming feature in the majority of the series was how constantly cold he was. This coldness can also cause freezing to his body after shock, as noted in RS2.
In the end of RS5, however, the genes in his body causing him to be so cold were drained by the alien leader Viz to be used in powering up a giant ice laser beam aimed at planet Earth. After the beam was used, Zack's new lack of low-temperature molecules led him only to have a very high temperature instead, to the point of spontaneous combustion.
In Riddle Transfer it is proved his last name is Kelvin, his favorite color is red, he hates dodgeball, he never ties his shoes, pigeons creep him out, and sneezes when he sniffs daisies because he is allergic to them. He also completely catches on fire when he sneezes and is given the title '"Human Fireplace" later in the series.
In RT2, Zack was see in the credits as a serviceman in a restaurant, serving Smiley and Phil. Interestingly enough, while he still has fire on his head, he has grown hair. He is also seen reading a newspaper while teleporting back to the classroom.
Zack is friends with Phred, Smiley and Phil throughout the series. but only in the Riddle School 5 and Riddle Transfer he was notably friends with them, but he proved that he just knew all three of them in the first.
- Zack's last name is a joke on the measurement of temperature, Kelvin, which is used for measuring extreme temperatures much like the ones in Zack's body.
- In Riddle School 4, Zack is shown to have hair when he is bald in the rest of the series.
- However, considering that was merely a dream sequence, the plot still makes sense. Zack somehow died in RS4 by crashing through a airplane,fell in a volcano and froze to death as explained in the same game he died in.
- However, some hair is visible on Zack's head in the Riddle Transfer games.
- While looking through a scrapbook of Phil and Smiley's life during the credits of Riddle Transfer 2, he grew hair as a waiter.
- JonBro announced that he was originally going to give Zack hair over time in the previously cancelled Riddle Transfer series.
- Zack is the only character with eyes that are not colored black, at least in Riddle School 4.
- Throught the series, his skin changes. in the 1st and 3rd games, his skin is a pale blue. In the 2nd game, he appears to be colder than he ever has been before, and in the 4th game, he looks warmer. In Riddle Transfer and Riddle School 5, his skin is back to how it was in the 1st game.
- The fact that he bursts in flames when he sneezes may be related to how he becomes completely frozen when he is in shock.
- Zack seems to be fine with burning, but he really hates being cold. This is probably because he has never been warm in his life.
- Zack was originally going to be set back at a normal temperature in the fifth Riddle Transfer game, which was later canceled. This cancel allowed Zack to be easily spotted as the waiter in the end of Riddle Transfer 2, serving Smiley and Phil
- Zack first had a light blue shirt in RS, then a blue shirt in RS2 then a light blue sweatshirt in RS3, then a dark blue sweatshirt in RS4 and finally a blue-grey shirt in RS5, RT1 and RT2
*FOOTNOTE - Zack's surname is a reference to the heat measurement Kelvin (K), which goes up similar to Celsius (C) but starts at absolute freezing (−273.15 °C)