Besides the titans, there are many other characters in the TBTTRAH storyline besides guest stars, special characters like Ihr and the Dark Lord, and the titans. This page contains all of them.
At the beginning, the Tahus were the rulers of the house. That is, of course, Tahu and Tahu Nuva. Of the two, Tahu is the most frequently appearing, though Tahu Nuva is always there to defend his precious windows whenever they get broken. His poor, poor beautiful glass windows. Both possess super bouncy shields that send their enemies flying upon impact, and it waqs Tahu who invented Piraka Baseball, which has from there evolved into many other games involving Bionicles using bats nearly two times their size.
Tahu is known for being the subject to the affection of Toa Kara, and has shown such feelings are at least partially mutual. In Aftermath, he calls a full on alert for the Zehvor to hunt down and return Kara after her being mysteriously kidnapped by 4-Mask.
Rahi are not a very important part of the storyline. However, they still appear every once and a while, and made their most important appearance in the first California trip in which Omega Turtle (at that time Stuffed Turtle) appeared for, though not the first time, the first time in the story. The Manas were unable to pass the metal detector, and Dekar saved his friend, a Nui-Rama, from an evil Waitress with the help of the Stuffed Turtle.
When the Barraki first appeared, they were hunting down toa in the pool, mistakenly believing at this time they could not breathe air, despite Nocturn revealing they were plastic. The six mostly, after failing to eat the Piraka, battled Hydraxon. They were present in the first California trip. After the early chapters, Carapar and Pridak were the most commonly appearing of the Barraki, with Pridak often being involved in evil plans and Carpar being his gullible fool, or decking the halls... literally.
The Piraka aren't really that important. They really don't do much... except Zaktann. Zaktann is an evil master mind, despite being dumb enough to search for the non-existant Voya-Nui. Like the Barraki, the Piraka are enemies of the jailor Hydraxon. Zaktann was the first person to begin mining in the Cofee Mines, after being addicted by MT, and he had Zamor sphere enslaved Matoran digging deep below the house. At this time, none knew that it was truely Xenon.