Haven't read the other works, who are they?
Hiiragi and Tomikawa from Hiiragi-Chan to Tomikawa-Chan.
Hiiragi is the one with long black hair. She's shy, anxious, and frequently overthinks things. At multiple times her behavior is compared to a scared wet cat.
Tomikawa is the one with blonde hair. She's an outgoing, vocal, headstrong girl with a genki affect and sparkly persona. With her bleached-blonde hair, endless energy, and love of sweets, she's most often compared to a golden retriever.
...except that's only the surface level. See, both Tomikawa and Hiiragi come from abusive family situations and are hiding no small amount of mental and emotional issues. This happy affect will sometimes be dropped in favor of a more realistic art style, darker colors, and multi-page updates that delve into their respective neuroses and pasts.
But at the end of the day they're good kids struggling with an unfair lot in life, and making the most of each other's company. Precious lil goobers who just need a helping hand.
The author refers to it as depression × lily, which I think is a very apt description.