A comprehensive study on the structure, nature of interaction, and properties of six ionic pairs of 1-butylpyridinium and 1-butyl-4-methylpyridinium cations in combination with tetrafluoroborate (BF), chloride (Cl), and bromide (Br) anions have been carried out using density functional theory (DFT). The anion-cation interaction energy (ΔE), thermochemistry values, theoretical band gap, molecular orbital energy order, DFT-based chemical activity descriptors [chemical potential (μ), chemical hardness (η), and electrophilicity index (ω)], and distribution of density of states (DOS) of these ion pairs were investigated. The ascendancy of the -CH substituent at the fourth position of the 1-butylpyridinium cation ring on the values of ΔE, theoretical band gap and chemical activity descriptors was evaluated. The ΔE values were negative for all six ion pairs and were highest for Cl containing ion pairs. The theoretical band gap value after -CH substitution increased from 3.78 to 3.96 eV (for Cl) and from 2.74 to 2.88 eV (for Br) and decreased from 4.9 to 4.89 eV (for BF). Ion pairs of BF were more susceptible to charge transfer processes as inferred from their significantly high η values and comparatively small difference in ω value after -CH substitution. The change in η and μ values due to the -CH substituent is negligibly small in all cases except for the ion pairs of Cl. Critical-point (CP) analyses were carried out to investigate the AIM topological parameters at the interionic bond critical points (BCPs). The RDG isosurface analysis indicated that the anion-cation interaction was dominated by strong H···X and C···X interactions in ion pairs of Cl and Br whereas a weak van der Waal's effect dominated in ion pairs of BF. The molecular electrostatic potential (MESP)-based parameter ΔΔV measuring the anion-cation interaction strength showed a good linear correlation with ΔE for all 1-butylpyridinium ion pairs (R = 0.9918). The ionic crystal density values calculated by using DFT-based MESP showed only slight variations from experimentally reported values.