ABSTRACT. We generalize the string functions C n,r (τ) associated with the coset sℓ(2) k /u(1) to higher string functions A n,r (τ) and B n,r (τ) associated with the coset W(k)/u(1) of the W -algebra of the logarithmically extended sℓ(2) k conformal field model with positive integer k. The higher string functions occur in decomposing W(k) characters with respect to level-k theta and Appell functions and their derivatives (the characters are neither quasiperiodic nor holomorphic, and therefore cannot decompose with respect to only theta-functions). The decomposition coefficients, to be considered "logarithmic parafermionic characters," are given by A n,r (τ), B n,r (τ), C n,r (τ), and by the triplet W(p)-algebra characters of the (p = k + 2, 1) logarithmic model. We study the properties of A n,r and B n,r , which nontrivially generalize those of the classic string functions C n,r , and evaluate the modular group representation generated from A n,r (τ) and B n,r (τ); its structure inherits some features of modular transformations of the higher-level Appell functions and the associated transcendental function Φ.