# import necessary libraries
import pandas as pd

# define your string and create series of it
string = 'data science and analysis'
series = pd.Series(list(string))

# replace missing values in series with NaN
series[series == ' '] = pd.NaT

# find frequency counts of every character
freq = series.value_counts(sort = True)

# find the least frequent character
least_frequent = freq.dropna().index[-1]

# replace missing values with least frequent character
results = series.fillna(least_frequent)

# print results
print(results.str.cat(sep=''))