Althea is from the Greek althaino ("to heal") — the mythological queen of Calydon in Homer's Iliad and the botanical name of the marsh mallow plant. Princess Althea of Calydon in Greek mythology — wife of Oeneus, mother of Meleager; her tragic fate after the Calydonian boar hunt is dramatized in Homer's Iliad Book 9 and Swinburne's Atalanta in Calydon (1865).
Althea does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Althea reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.