Hello Hela
Yes, you have to add the 's to "Tess". It seems that we may only leave off the s if the terminal sound is z:
1. Loch Ness's other monster.
2. Black Bess's younger sister.
3. The princess's pillow.
But:
4. Jones' house/Jones's house.
You might see or hear "Sophocles's" or "Socrates's"; and you would be a little more likely to see/hear "Marcus Aurelius's"; but the consensus seems to be that where a classical name refers to a classical character, you avoid the "apostrophe-s".
Perhaps it's a little too Germanic to sit happily with a Greek or Latin (or Hebrew) name. Though when a name is now an accepted English forename, the genitive seems more acceptable, e.g. "Marcus's", in reference to a modern person.
MrP