Rybnikova had been using a cable to charge her device while she bathed.
Speaking to reporters with Komsomolskaya Pravda, Rybnikova's older sister, Tatiana, revealed that the martial artist had recently been named godmother to her young daughter.
"My child adored her and called her 'nanny.' And now she is not with us anymore. Our hearts are torn up. It is so quiet and feels empty at home without her."
"She was a great person, the best and the most loved," a friend recalled of Rybnikova.
"She always dreamed of success in sports. We wanted to go together with her to another city and study there. Everybody loved her. She was sociable, kind and beautiful."
Another close friend said Rybnikova "dreamed of becoming a world champion."
Rybnikova was a champion fighter in pankration, a form of boxing and wrestling dating back to Ancient Greece. She won the all-Russian national championship two months ago and was selected for her country's national team.
Watch Rybnikova compete in this video.
Teenage martial arts champion is electrocuted in bath by her charging iPhone Mirror Onlinwww.youtube.com
"A tragic accident has taken the life of our champion, friend, and student Irina Rybnikova, 15," Rybnikova's sports federation said in a statement.
"She was a candidate for Master of Sports in pankration, our beloved girl. Rest in peace."
The star athlete's death also swept through social media; the incident prompted several people to issue stern warnings, saying the tragic accident could have easily been avoided.
Reporter Victor Bolaños also lamented the nature of Rybnikova's death, saying that our connections to our cell phones come with "latent risks."
These risks don't unnerve us, he added, noting that we practically sleep on top of our phones when we get into our beds at night.
"Water is a good conductor for a current, this is why there was a short circuit when the phone fell into the water," said Yury Agrafonov, the head of the radio-electronic department of Irkutsk State University.
"If the phone had not been plugged in to 220 volts, the tragedy would not have happened."
A study conducted by FU Berlin between 1995 and 1999 on death by electrocution in bathtubs found 41 cases (31 of these involved hairdryers). 16 of these cases showed cutaneous marks, which are signs consistent with electric shock.
Signs of drowning were found in seven cases and two of these showed cutaneous marks, suggesting the electric shock did not kill individuals outright.
It could be "that the current stops your heart," Ian Lang, an electronic engineer, told Quora last year.
"It could be that the shock causes drowning, or it could be that the shock causes you to flail about perhaps slipping and hitting your head."
"Either way, it appears to be a rare phenomenon," he concluded.
"The best idea is to keep electrical appliances away, and in the UK you're not allowed any sockets in a bathroom."