The St. Christopher medal is a small metal medallion which Jonas Kahnwald found at the lake and later gave to Martha Nielsen. He is first seen with it at the beginning of Season 2, taking it with him after waking up in the derelict Kahnwald home.
In Christian tradition, Saint Christopher (Latin: Christophorus; German: Christoph) was a third-century martyr. He is a patron saint of travellers, unmarried men, and sailors among many others, and is invoked for protection against storms, floods, and lightning. The church in Winden is dedicated to him.
According to legend, as a youth, he sought to follow the greatest king in the world. When the king crossed himself at the name of the devil, he decided the devil was stronger and sought to follow the devil. But finding that the devil feared Christ, Christopher decided to follow Christ instead. His name is derived from Greek, meaning "Christ-bearer," after another legend in which he ferries a small child across a raging river, who is then revealed to be Jesus.
Devout travellers may carry medallions of St. Christopher for good luck; like Jonas', they depict the saint carrying a child on his back.
The St. Christopher medal appears for the first time chronologically in when Egon gives Hannah the medal. She carries it with her and is also wearing it when she goes to an abortion clinic. There, she meets Helena, Katharina's mother as a child, who shows some interest in Saint Christopher. When Helena is gone, Hannah decides not to abort her baby but leaves the medal for Helena.
Now Helena has the medal and keeps it, after which we see the medal again when Katharina visits the psychiatric ward and talks to her mother. When Helena fights Katharina at the lake, Katharina snatches the medal off of Helena, and it falls in the sand where Helena forgets about it.
In June of 2019, Jonas, Martha, Bartosz and Magnus go swimming in the lake. Jonas finds the medal in the sand, in the same spot where Katharina left it..