Foil
Character Role Analysis
Foil: Theodora & Luke
Theodora and Luke are foils for Eleanor. That is, both characters act as a sort of mirror for her. Well, a funhouse mirror might be more precise. Through them, we see Eleanor from a different perspective than we would otherwise.
When Luke and Eleanor talk at the summerhouse, Eleanor and Luke discuss loneliness, while inwardly Eleanor thinks Luke is a "vain" and "selfish man" (6.2, 11). Luke may be vain and selfish, but if you think about it, so is Eleanor herself. Her negative reaction to these qualities might just tell us more about Eleanor than about Luke.
Theodora is more of an evolving foil. At first, Eleanor seems to want to obtain the attributes of Theodora: she thinks "that it had been a very long time since she had dressed to look like a stray sunbeam, or been so hungry for breakfast" (4.12). Remember: Theodora dressed in yellow on their first day at Hill House, and she's always starving for breakfast in the morning. Eleanor is adopting Theodora's traits.
But as their time in Hill House continues, Eleanor begins to resent Theodora, even going so far as to want to "batter her with rocks" (5.158). The change in her relationship with Theodora begins to signify Eleanor's change into someone belonging to Hill House. It seems that Hill House wants to destroy its residents, and we imagine Theodora would be good and destroyed if battered with a large enough rock.