Tefnut is the Egyptian goddess of the rain. She has connections to both the sun and the moon, but her primary duty is rain/moisture.
Tefnut was shown as a grown woman with the head of a lioness. She carried a scepter and ankh and sat on a throne. A full solar disk sat atop her head, circled by two cobras.
The head of Tefnut is a lioness because the sun god Atum created Tefnut and her brother Shu as lion cubs. Thus Atum is considered their father. Shu is the god of air and also Tefnut’s husband. The pair spawned the god of earth, Geb, and Nut the goddess of the sky. All five of these deities were a part of the Ennead, a group of nine original gods worshiped at the birthplace of the gods, Heliopolis. (The other four are the children of Nut and Geb: Osiris, Isis, Set, and Nephthys.)
Tefnut was an important deity to the ancient Egyptians. Their country is mostly desert, with only the Nile to give any kind of sustainability to human civilization. Water was incredibly important, and the goddess of moisture was likewise important.
She shared a role with other goddesses as the protector of the sun god and an Eye of Ra. This is where her connection to the sun and moon come in. As a goddess of moisture she can be the absence of moisture (dried from the sun) or the abundance of moisture (connected to the moon). While Tefnut shared the sun aspect with other goddesses, she was actually considered to be both the Eyes of Ra, the left (moon) and the right (sun), and seems to have held the moon aspect in her own right.
One myth says that Tefnut became angry with her father Atum/Ra while he ruled on earth. She was so angry that she went to Nubia and took all the moisture of Egypt with her. She rampaged across Nubia as a lioness while Egypt dried up. Atum/Ra sent Thoth and Shu after her to placate her and bring her back. They succeeded, and on her way back Tefnut visited every village in Egypt and brought great rejoicing.
Another myth says that Tefnut and Shu went into the waters of Nun (chaos). Atum/Ra thought he had lost them forever, and sent his eye to find them. But Tefnut and Shu returned from chaos, and Atumwas so happy to find them alive that he wept. His tears became the first humans.
Light and Dark Side
Considering the desert that is Egypt, a goddess of moisture is something of a fertility deity. She helps to not only bring life but sustain it. In the last myth, where Amen’s tears create the first humans, it could be argued that actually this is due to Tefnut–tears are moisture, and she would have had control over that moisture. Her life-giving/fertility nature gives her a boost to the Light side of the deity spectrum.
However, in the tradition of ancient deities, Tefnut is somewhat petty at times. When she had a spat with her father, she took it out not only on him but all of Egypt, harming innocents in her quarrel with Atum. Then she rampages through a country that has done her no harm and had no part in the original quarrel in the first place. Such petty actions, and the fact that she harms innocents with her actions, gives her a Darker aspect. Another Dark side is the ability to control the moisture so that there is drought or flood. We have already seen her do this–she caused a drought in Egypt and a flood in Nubia. She likely would be prone to such things again (and since floods and droughts are common natural events, she probably did).
Overall, Tefnut is a Light goddess, because of her ability to give life. But the Dark aspects are worth considering as well.